Contact opening metrology

ABSTRACT

A method for process monitoring includes receiving a sample having a first layer that is at least partially conductive and a second layer formed over the first layer, following production of contact openings in the second layer by an etch process, the contact openings including a plurality of test openings having different, respective transverse dimensions. A beam of charged particles is directed to irradiate the test openings. In response to the beam, at least one of a specimen current flowing through the first layer and a total yield of electrons emitted from a surface of the sample is measured, thus producing an etch indicator signal. The etch indicator signal is analyzed as a function of the transverse dimensions of the test openings so as to assess a characteristic of the etch process.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 10/209087, filed Jul. 30, 2002, which is assignedto the assignee of the present patent application and is incorporatedherein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to microfabrication ofholes and trenches, including vias, damascene structures and the like,in semiconductor device manufacturing, and specifically to monitoring ofcontact holes produced on semiconductor wafers.

BACKGROUND OF THE INVENTION

[0003] Contact hole production is a common step in semiconductorintegrated circuit manufacturing. The contact holes are typically usedto make electrical connections to a semiconductor or metal layer throughan overlying non-conducting (dielectric) layer, such as an oxide layer,or partially-conductive layer. In order to produce contact holes, alayer of photoresist is deposited on the wafer surface. The photoresistis exposed to ultraviolet or other radiation, hardened and developed inorder to form a “mask” over the wafer, with openings at the locations ofthe contact holes. Then the wafer is transferred to an etch station toform the contact holes through the non-conducting layer down to thesemiconductor layer. The photoresist mask is then removed, and thecontact holes are filled with metal. A similar process is used inproducing trenches or vias in the wafer surface.

[0004] In order to ensure consistent device performance, the depth,width and bottom surface cleanliness of contact openings must becarefully controlled. (In the context of the present patent applicationand in the claims, the term “contact openings” refers to all structuresof the type described above, including both contact holes, vias andtrenches. Certain techniques for inspecting contact openings andmonitoring their production, however, are described by way of examplewith specific reference to contact holes.) Deviations in the dimensionsof contact openings can lead to variations in the contact resistance.These variations can have a serious impact on device performance and canlead to loss of process yield. The manufacturing process must thereforebe carefully monitored and controlled, in order to detect deviations information of contact openings as soon as they occur and to takecorrective action to avoid the loss of costly wafers in process.

[0005] It is known in the art to use a scanning electron microscope(SEM) to inspect contact holes and other contact openings. Theprinciples of the SEM and its use in microanalysis of semiconductordevice structures are described, for example, by Yacobi et al., inChapter 2 of Microanalysis of Solids (Plenum Press, New York, 1994),which is incorporated herein by reference. Because contact holes aretypically much deeper than they are wide, a special high aspect ratio(HAR) imaging mode is used, as described by Yacobi et al. Open contactholes, which reach down through the dielectric layer to thesemiconductor below, appear bright in the image, while closed holes,which do not fully expose the semiconductor layer, are dim.

[0006] HAR techniques using a SEM are time-consuming and costly toimplement, and they become impractical at very high aspect ratios(roughly >10), which are used in some integrated circuits, such as DRAM.They are also not capable of distinguishing between different types ofblockage that can cause contact holes to be closed (for example,under-etching of the holes, as opposed to deposition of residues in thebottoms of the holes). Furthermore, HAR imaging techniques can generallybe used only after the photoresist mask has been cleaned from the wafersurface. Consequently, there is no possibility of continuing the etchingprocess if it is discovered upon inspection that the contact holes havebeen underetched.

[0007] An alternative method for contact hole inspection is described byYamada et al., in “An In-Line Process Monitoring Method Using ElectronBeam Induced Substrate Current,” in Microelectronics-Reliability 41:3(March 2001), pages 455-459, which is incorporated herein by reference.The substrate current in an electron beam system, also known as thespecimen current, absorbed current or compensation current, is definedas the absorbed current that flows or would flow from the primaryelectron beam to ground (earth) via the specimen (i.e., via the wafer).In other words, the specimen current is equal to the difference betweenthe primary beam current (i.e., the current of electrons in the electronbeam that irradiates the specimen in the system) and the total yield ofelectrons emitted from the surface of the specimen due to secondary andbackscattered electrons (adjusted for any local charging effects or timeconstants). The specimen current can be either positive or negative,depending on whether the energy of the primary electron beam is in thepositive- or negative-charging domain of the specimen. (The phenomena ofpositive and negative charging by e-beam irradiation are described inthe above-mentioned reference by Yacobi et al.) Yamada et al. directedan electron beam at single contact holes and groups of holes in a SiO₂surface layer overlying a silicon substrate, and measured the resultantspecimen current. They found that the specimen current was a goodindicator of hole-bottom oxide thickness, as well as of the holediameter.

[0008] Yamada et al. describe further aspects of contact holemeasurement in U.S. patent application Publication No. US 2002/0070738A1, whose disclosure is incorporated herein by reference. Semiconductordevices are inspected by measuring the specimen current in an area of asample having no contact holes as a background value, and comparing thisvalue to the current measured in the area of a hole. A current waveformis automatically evaluated in order to determine whether the measurementis indicative of a defect of the device or of manufacturing equipmentused in producing the device.

SUMMARY OF THE INVENTION

[0009] The present invention provides improved methods and systems forassessing characteristics of contact openings and other openings formedin dielectric or partially-conductive layers on a sample, andparticularly for monitoring the quality of contact openings, such ascontact holes, as well as vias or trenches, in production ofsemiconductor devices.

[0010] In embodiments of the present invention, a charged particle beam,typically an electron beam, characterized by a primary beam current, isdirected to irradiate areas of a sample in which contact openings orother openings are etched. The etch state of the contact openings isdetermined by measuring an etch indicator signal generated by the samplein response to the electron beam. The etch indicator signal is typicallybased on the overall specimen current flowing through a single openingor an array of openings in the sample circuit. Alternatively oradditionally, the etch indicator signal may be based the total yield ofsecondary and backscattered electrons emitted from the surface of thesample. The specimen current can be deduced from the difference betweenthe primary beam current and the total electron yield. Although certainembodiments are described hereinbelow with specific reference tospecimen current, the principles of these embodiments may generally beadapted to work with etch indicator signals based on the total yield ofsecondary and backscattered electrons, instead of or in addition to thespecimen current.

[0011] The “etch state” or “etch quality” of contact openings, as usedin the context of the present patent application and in the claims,refers to one or more of a set of characteristics of the contact orother openings. These characteristics generally include the remaining(residual) thickness of any dielectric material at the bottom of thehole, and they may also include the contact, trench or via depth and/orwidth, the presence or absence of a residue inside the openings, and anypunch through an etch stop layer to damage underlying copper in adamascene structure. The measurement of specimen current is indicativeof the diameter of the bottom of the contact hole, where contact is madewith the underlying layer, unlike HAR SEM imaging, which shows mainlythe diameter at the upper end of the contact hole. The diameter of thebottom of the contact hole is a critical dimension, in terms of itseffect on the resistance of the contact formed when the hole is filledwith metal.

[0012] The etch indicator signal may be measured using a narrow particlebeam, to irradiate the area of a single contact opening. Alternatively,a group of contact openings may be irradiated simultaneously, typicallyusing a defocused or rastered focused beam, to give an enhanced signal,as described in the above-mentioned U.S. patent application Ser. No.10/209,087.

[0013] In some embodiments of the present invention, a calibrationprocedure is used to determine an absolute threshold specimen currentfor a given, nominal contact hole diameter. The actual specimen currentis then measured by irradiating a number of different contact holes ofthe nominal diameter, distributed over the surface of the sample. If themeasured magnitude of the specimen current due to irradiation of thesample in the area of a given contact hole is equal to or greater thanthe threshold, the contact hole is considered to have been etchedsufficiently. If the measured specimen current is significantly lessthan the threshold, however, the contact hole is considered to beunderetched, indicating that a process fault may have occurred.

[0014] Another calibration procedure is used to determine a relativethreshold for the specimen current. The relative threshold defines amaximal non-uniformity of specimen current measurements, made on thesame type of contact hole in different locations on the sample.Variations in the specimen current measured at the different locationsthat are greater than the threshold are considered to be indicative of aproblem in the manufacturing process.

[0015] In some embodiments of the present invention, a test structure isformed on the sample, either on a wafer scribe line or in-die, for usein assessing process quality. The test structure can, for example,comprise an array of contact openings of varying transverse dimension,such as contact holes of graduated diameter, with some openings that arewider than the nominal diameter of functional contact holes to be formedon the sample, and others that are narrower. (In the context of thepresent patent application and in the claims, “transverse” refers to adirection or dimension perpendicular to the depth dimension of a contactopening.) During etching of the sample, the depths of the contact holeswill increase at a rate that is roughly proportional to their diameters.When the etch process is adjusted properly, the specimen currentmeasured for the test holes in the vicinity of the nominal diameter orgreater should be high, indicating complete etching of the dielectric.The specimen current measured for the small-diameter holes may bemarkedly lower, indicating incomplete etching of these holes. The teststructure thus provides a reading of the variation of etch depth as afunction of hole diameter. Changes in this reading may be used to detectincipient process defects such as underetching, before the defectsbecome serious enough to affect the quality of the functional contactholes.

[0016] As another example, the test structure may comprise both denseand sparse arrays of contact holes having the same diameter. The etchrate of contact holes often is a function of contact hole density, dueto a micro-loading effect when contact holes are closely spaced. Thus,in general, the etch rate is substantially lower in the dense contacthole arrays than in the sparse arrays. The spacing of the contact holesin dense and sparse arrays in the test structure is typically chosen torepresent limiting cases of actual contact hole spacing for in-diepatterns. Alternatively or additionally, the density of the contactholes can be determined from prior knowledge of the etch process window.Therefore, by measuring the etch indicator signal with respect tocontact holes in the dense and sparse arrays, it is possible to detectetch problems that may occur within in-die patterns due tomicro-loading.

[0017] In further embodiments of the present invention, novel testconfigurations are used to enhance the strength or sensitivity of theetch indicator signal for a given particle beam current and contact holesize. These test configurations are useful in particular to enhancesensitivity to very thin layers or remaining dielectric at the bottom ofthe contact hole. In one of these embodiments, the particle beamirradiates the surface of the sample at a non-normal angle, i.e., withat least a slight tilt. As a result, the energetic primary beam strikesthe side walls of the contact holes, rather than the bottom. The surfaceof the sample is negatively precharged, so that secondary electronsemitted from the side walls and upper edge of the contact holes aredriven down toward the bottom of the holes. The secondary electrons,however, are substantially less energetic than the electrons in theprimary beam. Therefore, the secondary electrons are less able than theprimary electrons to penetrate through thin residue layers that mayremain at the bottom of the contact holes. As a result, the measurementof specimen current using an angled particle beam can, under someconditions, provide a more sensitive indicator of etch state than can beachieved using a conventional, normal-incidence beam.

[0018] The angled beam may be used to enhance the sensitivity ofspecimen current measurements in other applications, as well, as will beapparent to those skilled in the art. For example, the angled beam maybe used to measure punch-through of contact hole side walls, which leadsto current leakage through the side walls to nearby polysiliconstructures. As another example, contact holes produced at the peripheryof a silicon wafer may be tilted due to the effect of fringing fields inthe dielectric etch process. The electron beam may be angled so that theelectrons still strike the bottom of these tilted contact holes or toensure the beam hits the contact hole side wall at a desired angle.

[0019] In another embodiment, the sample is irradiated simultaneously bya charged particle beam and by electromagnetic radiation, i.e., by abeam of photons, typically a beam of visible, near-infrared orultraviolet light. This technique is useful, for example, in assessingthe etch quality of contact holes used to contact P-N junctions infunctional dice (or otherwise connected to P-N junctions), which arefabricated in a semiconductor wafer. When such a junction is biased bycharge from a charged particle beam alone, little or no specimen currentmay flow through to the semiconductor substrate if the charge-inducedvoltage reverse-biases the junction (since the junction acts as anon-conducting reverse-biased diode). If the junction is irradiated withlight at a photon energy greater than the semiconductor bandgap energy,however, electron-hole pairs will be created at the P-N junction and inthe substrate, so that a significant specimen current may flow throughthe P-N junction. The combination of particle beam and electromagneticirradiation can also be used to measure other aspects of devicesproduced on semiconductor wafers, particularly front-end devicestructures, as will be apparent to those skilled in the art.

[0020] As noted above, in measurements of specimen current flowingthrough contact holes, it is frequently advantageous to negatively biasthe upper surface of the sample, i.e., the surface on which the electronbeam is incident. In systems known in the art, the negative bias iscreated by operating the electron beam at high energy, in the negativecharging domain (i.e., the energy range in which the total yield ofbackscattered and secondary electrons from the wafer is less than theprimary electron beam current), in order to precharge the surface.High-energy irradiation, however, can cause damage to the sample.Therefore, in some embodiments of the present invention, an electrodenear the surface is used to apply a negative bias potential while thesurface is irradiated by the electron beam. The bias potential causesthe secondary electrons emitted from the surface to return to thesurface, thus creating a net negative precharge, without the need forhigh-intensity, high-energy irradiation of the surface as in systemsknown in the art.

[0021] There is therefore provided, in accordance with an embodiment ofthe present invention, a method for process monitoring, including:

[0022] receiving a sample having a first layer that is at leastpartially conductive and a second layer formed over the first layer,following production of contact openings in the second layer by an etchprocess, the contact openings including a plurality of test openingshaving different, respective transverse dimensions;

[0023] directing a beam of charged particles to irradiate the testopenings;

[0024] measuring, in response to the beam, at least one of a specimencurrent flowing through the first layer and a total yield of electronsemitted from a surface of the sample, thus producing an etch indicatorsignal; and

[0025] analyzing the etch indicator signal as a function of thetransverse dimensions of the test openings so as to assess acharacteristic of the etch process.

[0026] In an aspect of the invention, analyzing the etch indicatorsignal includes assessing a residual thickness of the dielectric layerat a bottom of the test openings as a function of the transversedimensions. In one embodiment, the test openings include a first openinghaving a first transverse dimension, and at least a second openinghaving a second transverse dimension that is less than the firsttransverse dimension, and the method includes controlling the etchprocess, in response to the etch indicator signal, so that the firstopening is sufficiently deep to reach the first layer, while at leastthe second opening is not sufficiently deep to reach the first layer. Inanother embodiment, the test openings further include a third opening,having a third transverse dimension intermediate the first and secondtransverse dimensions, and analyzing the etch indicator signal includesdetecting a potential process defect when the etch indicator signalindicates that the third opening is not sufficiently deep to reach thefirst layer.

[0027] In another aspect of the invention, the sample may have a barrierlayer formed between the first and second layers, and assessing theresidual thickness may include analyzing the etch indicator signal afteretching the second layer in order to assess an integrity of the barrierlayer, and then analyzing the etch indicator signal after etching thebarrier layer, typically in order to verify that at least some of thecontact openings have been etched through the barrier layer to the firstlayer.

[0028] In still another aspect of the invention, analyzing the etchindicator signal includes assessing a critical dimension of a bottom ofthe test openings as a function of the transverse dimensions.

[0029] Optionally, analyzing the etch indicator signal includesmeasuring a beam current of the beam of charged particles, and analyzinga ratio of the etch indicator signal to the beam current. Alternatively,measuring at least one of the specimen current and the total yieldincludes measuring the total yield of the electrons emitted from thesurface of the sample and further includes measuring a primary currentof the beam, and taking a difference between the primary current and thetotal yield to determine the etch indicator signal.

[0030] In a disclosed embodiment, the plurality of test openingsincludes multiple groups of the test openings in respective test areas,which are distributed in different locations across the sample, anddirecting the beam includes positioning at least one of the beam and thesample so as to irradiate each of at least two of the test areas inturn. Analyzing the etch indicator signal may include evaluating avariation of the etch indicator signal across the sample so as to assessa uniformity of the etch process.

[0031] Typically, directing the beam includes operating the beam so asto precharge a surface of the sample in proximity to the test openings,so as to facilitate measurement of the specimen current.

[0032] In an aspect of the invention, the sample includes asemiconductor wafer, and the contact openings include at least one ofcontact holes, trenches and vias. At least some of the contact openingsnot included in the plurality of test openings may belong to multiplemicroelectronic circuits on the wafer, wherein the circuits areseparated by scribe lines, and the test openings are located on one ofthe scribe lines.

[0033] In another aspect of the invention, receiving the sample includesreceiving the sample with a photoresist layer overlying the secondlayer, the photoresist layer having been used in etching the contactopenings, and analyzing the etch indicator signal includes monitoringthe etch indicator signal while irradiating the test area, prior toremoving the photoresist layer. The method may include, if the etchindicator signal indicates that a residual thickness of the second layerat a bottom of one or more of the test openings is greater than apredetermined limit, further etching the second layer using thephotoresist layer so as to increase the depth.

[0034] In one embodiment, analyzing the etch indicator signal includesdetecting a residue within the contact openings, and the methodirradiating the sample with the beam of charged particles so as toremove the residue.

[0035] Optionally, directing the beam includes directing a pulsed beamof the charged particles to irradiate the test openings, and measuringat least one of the specimen current and the total yield of electronsincludes measuring a time variation of the specimen current bycapacitive coupling to the sample.

[0036] There is also provided, in accordance with an embodiment of thepresent invention, a method for process monitoring, including:

[0037] receiving a sample having a first layer that is at leastpartially conductive and a second layer formed over the first layer,following production of contact openings in the second layer by an etchprocess, the contact openings including at least first and second arraysof test openings, characterized by different, respective first andsecond spacings between the test openings in the first and secondarrays;

[0038] directing a beam of charged particles to irradiate the testopenings;

[0039] measuring, in response to the beam, at least one of a specimencurrent flowing through the first layer and a total yield of electronsemitted from a surface of the sample, thus producing an etch indicatorsignal; and

[0040] analyzing the etch indicator signal as a function of the spacingsof the arrays of the test openings so as to assess a characteristic ofthe etch process.

[0041] In an aspect of the invention, analyzing the etch indicatorsignal includes assessing a residual thickness of the dielectric layerat a bottom of the test openings as a function of the spacings.Typically, the first spacing is substantially greater than the secondspacing, and the method includes controlling the etch process, inresponse to the etch indicator signal, so that the test openings in thefirst array are sufficiently deep to reach the first layer, while thetest openings in the second array are not sufficiently deep to reach thefirst layer.

[0042] There is additionally provided, in accordance with an embodimentof the present invention, a method for monitoring a process carried outon a sample, the method including:

[0043] directing a beam of charged particles to irradiate the samplealong a beam axis that deviates substantially in angle from a normal toa surface of the sample;

[0044] measuring, in response to incidence of the beam on the sample, aspecimen current flowing through the sample; and

[0045] analyzing the specimen current so as to assess a characteristicof the process.

[0046] Typically, the sample has a first layer that is at leastpartially conductive and a second layer formed over the first layer, andthe process includes an etch process, which is applied to the sample soas to produce contact openings in the second layer, and directing thebeam includes irradiating the contact openings, and analyzing thespecimen current includes assessing the etch process. Some of thecontact holes may be characterized by a tilt relative to the normal tothe surface, and directing the beam may then include angling the beam soas to compensate for the tilt.

[0047] Typically, the contact openings have side walls and a bottom, anddirecting the beam may additionally or alternatively include angling thebeam so that more of the charged particles strike the side walls thanstrike the bottom. In an aspect of the invention, the contact openingsare characterized by an aspect ratio, and directing the beam includesaligning the beam axis at an angle that deviates from the normal to thesurface by at least an arctangent of an inverse of the aspect ratio.

[0048] There is further provided, in accordance with an embodiment ofthe present invention, a method for process monitoring, including:

[0049] directing a beam of charged particles to irradiate a surface of asample, whereby electrons are emitted from the surface;

[0050] applying an electric field in a vicinity of the surface, so as tocause at least a portion of the emitted electrons to return to thesurface, thereby generating a negative precharge at the surface; and

[0051] receiving a signal produced by the sample in response to the beamand the negative precharge.

[0052] Typically, the sample has a first layer that is at leastpartially conductive and a second layer formed over the first layer, andthe negative precharge is formed on the surface of the dielectric layer.

[0053] In an aspect of the invention, directing the beam includesoperating the beam during a precharging interval so as to generate thenegative precharge at the surface, and then operating the beam after theprecharging interval so as to generate the signal. Typically, operatingthe beam during the precharging interval includes setting the beamsource so that electrons have an energy in a positive charging domain ofthe surface of the sample.

[0054] There is moreover provided, in accordance with an embodiment ofthe present invention, a method for testing a semiconductor device,including:

[0055] irradiating a junction in the semiconductor device with a firstbeam including electromagnetic radiation;

[0056] irradiating the device with a second beam including chargedparticles, so that at least some of the charged particles are incidenton the junction substantially simultaneously with the electromagneticradiation; and

[0057] measuring, in response to incidence of the first and second beamson the junction, a property of the device.

[0058] In an aspect of the invention, measuring the property includesforming an electronic image of the device.

[0059] In another aspect of the invention, the junction includes asemiconductor material, and irradiating the junction with the first beamincludes irradiating the junction with photons having an energy greaterthan or equal to a bandgap of the semiconductor material. Typically, thejunction includes a P-N junction.

[0060] Additionally or alternatively, measuring the property includesmeasuring a current flowing through the device, wherein a dielectriclayer is formed over the junction, and a contact hole is formed throughthe dielectric layer in order to contact the junction, and whereinirradiating the junction with the first and second beams includesirradiating an interior of the contact hole, and wherein measuring thecurrent includes assessing a characteristic of the contact hole based onthe current. Typically, assessing the characteristic includes assessinga suitability of the contact hole to make a conductive electricalcontact with the junction.

[0061] There is furthermore provided, in accordance with an embodimentof the present invention, a method for process monitoring, including:

[0062] receiving a sample having a first layer that is at leastpartially conductive and a second layer formed over the first layer,following production of contact openings in the second layer by an etchprocess;

[0063] directing a beam of charged particles to irradiate one or more ofthe contact openings;

[0064] measuring a primary current of the beam and a total yield ofelectrons emitted from a surface of the sample in response incidence ofthe beam on the contact openings; and

[0065] analyzing a relation between the primary current and the totalyield of the electrons so as to assess a characteristic of the etchprocess.

[0066] Analyzing the relation may include analyzing a difference betweenthe primary current and the total yield or, additionally oralternatively, analyzing a ratio between the primary current and thetotal yield.

[0067] There is also provided, in accordance with an embodiment of thepresent invention, a method for process monitoring of a sample having afirst layer that is at least partially conductive and a second layerformed over the first layer, wherein contact openings are formed in thesecond layer by an etch process, the method including:

[0068] determining, for a given set of characteristics of the contactopenings, a threshold level of an etch indicator signal, which isproduced by measuring at least one of a specimen current flowing throughthe first layer and a total yield of electrons emitted from a surface ofthe sample in response to irradiation of the contact openings by a beamof charged particles;

[0069] directing the beam of charged particles to irradiate each of aplurality of the contact openings that have the given set ofcharacteristics and are disposed at different, respective positions overa surface of the sample;

[0070] determining, in response to the beam, the etch indicator signalproduced at each of the respective positions of the plurality of thecontact openings; and

[0071] comparing the etch indicator signal produced at the respectivepositions to the threshold level so as to assess a characteristic of theetch process.

[0072] Typically, comparing the etch indicator signal includesdetermining, if an absolute magnitude of the specimen current fallsbelow the threshold level by more than a predetermined margin, that atleast some of the contact openings are underetched.

[0073] Additionally or alternatively, determining the threshold levelincludes finding the level of the etch indicator signal that correspondsto etching of the contact openings through the second layer to exposethe first layer within the opening. In a disclosed embodiment, findingthe level includes calibrating the threshold level in a procedureperformed on a test sample, for subsequent application in assessing thecharacteristic of the etch process performed on other samples.Typically, calibrating the threshold level includes making measurementsof the etch indicator signal generated by the test sample, and comparingthe measurements to at least one of a cross-sectional profile of thecontact openings in the test sample and a conductivity of electricalcontacts made through the contact openings in the test sample.

[0074] In an embodiment of the invention, the sample has a barrier layerformed between the first and second layers, and finding the level of theetch indicator signal includes finding a first level that corresponds toetching of the contact openings through the second layer to expose thebarrier layer, and finding a second level that corresponds to etching ofthe contact openings through the barrier layer to expose the first layerwithin the openings. Typically, comparing the etch indicator signalincludes analyzing the etch indicator signal after etching the secondlayer in order to assess an integrity of the barrier layer, and thenanalyzing the etch indicator signal after etching the barrier layer inorder to verify that at least some of the contact openings have beenetched through the barrier layer to the first layer.

[0075] The method may include evaluating a variation of the etchindicator signal across the sample so as to assess a uniformity of theetch process, and signaling that a process fault has occurred if thevariation of the etch indicator signal across the sample is greater thana predetermined maximum.

[0076] There is additionally provided, in accordance with an embodimentof the present invention, a method for process monitoring of a samplehaving a first layer that is at least partially conductive and a secondlayer formed over the first layer, wherein contact openings are formedin the second layer by an etch process, the method including:

[0077] directing a beam of charged particles to irradiate each of aplurality of the openings that share a given set of characteristics andare disposed at different, respective positions across the sample;

[0078] measuring at least one of a specimen current flowing through thefirst layer and a total yield of electrons emitted from a surface of thesample in response to irradiation of the contact openings by the beam ofcharged particles, thus producing an etch indicator signal as a functionof the respective positions of the plurality of the openings; and

[0079] evaluating a variation of the etch indicator signal across thesample so as to assess a uniformity of the etch process.

[0080] In an aspect of the invention, evaluating the variation includesdetermining that a process fault has occurred if the variation of theetch indicator signal across the sample is greater than a predeterminedmaximum.

[0081] There is further provided, in accordance with an embodiment ofthe present invention, apparatus for etching a sample having a firstlayer that is at least partially conductive and a second layer formedover the first layer, contact openings having been created in the secondlayer by an etch process, the contact openings including a plurality oftest openings having different, respective transverse dimensions, theapparatus including:

[0082] a test station, which includes:

[0083] a particle beam source, which is adapted to direct a beam ofcharged particles to irradiate the test openings; and

[0084] a current measuring device, which is coupled to measure, inresponse to the beam, at least one of a specimen current flowing throughthe first layer and a total yield of electrons emitted from a surface ofthe sample, thus producing an etch indicator signal; and

[0085] a controller, which is adapted to analyze the etch indicatorsignal as a function of the transverse dimensions of the test openingsso as to assess a characteristic of the etch process.

[0086] There is moreover provided, in accordance with an embodiment ofthe present invention, apparatus for etching a sample having a firstlayer that is at least partially conductive and a second layer formedover the first layer, contact openings having been created in the secondlayer by an etch process, the contact openings including at least firstand second arrays of test openings, characterized by different,respective first and second spacings between the test openings in thefirst and second arrays, the apparatus including:

[0087] a test station, which includes:

[0088] a particle beam source, which is adapted to direct a beam ofcharged particles to irradiate the test openings; and

[0089] a current measuring device, which is coupled to measure, inresponse to the beam, at least one of a specimen current flowing throughthe first layer and a total yield of electrons emitted from a surface ofthe sample, thus producing an etch indicator signal; and

[0090] a controller, which is adapted to analyze the etch indicatorsignal as a function of the spacings of the arrays of the test openingsso as to assess a characteristic of the etch process.

[0091] There is furthermore provided, in accordance with an embodimentof the present invention, apparatus for monitoring a process carried outon a sample, the apparatus including:

[0092] a particle beam source, which is adapted to direct a beam ofcharged particles to irradiate the sample along a beam axis thatdeviates substantially in angle from a normal to a surface of thesample;

[0093] a current measuring device, which is coupled to measure, inresponse to the beam, a specimen current flowing through the sample; and

[0094] a controller, which is adapted to analyze the specimen current soas to assess a characteristic of the etch process.

[0095] There is also provided, in accordance with an embodiment of thepresent invention, apparatus for process monitoring, including:

[0096] a particle beam source, which is adapted to direct a beam ofcharged particles to irradiate a surface of a sample, whereby electronsare emitted from the surface;

[0097] a bias electrode, which is adapted to apply an electric field ina vicinity of the surface, so as to cause at least a portion of theelectrons emitted during the precharging interval to return to thesurface, thereby generating a negative precharge at the surface; and

[0098] a current measuring device, which is coupled to receive a signalproduced by the sample in response to the beam and the negativeprecharge.

[0099] There is additionally provided, in accordance with an embodimentof the present invention, apparatus for testing a semiconductor device,including:

[0100] a radiation source, which is adapted to irradiate a junction inthe semiconductor device with a first beam including electromagneticradiation;

[0101] a particle beam source, which is adapted to irradiate the devicewith a second beam including charged particles, so that at least some ofthe charged particles are incident on the junction substantiallysimultaneously with the electromagnetic radiation; and

[0102] a measuring element, which is adapted to measure, in response toincidence of the first and second beams on the junction, a property ofthe device.

[0103] There is further provided, in accordance with an embodiment ofthe present invention, apparatus for monitoring an etch process appliedto a sample having a first layer that is at least partially conductiveand a second layer formed over the first layer, following production ofcontact openings in the second layer by the etch process, the apparatusincluding:

[0104] a particle beam source, which is adapted to direct a beam ofcharged particles to irradiate one or more of the contact openings;

[0105] a beam current detector, for detecting a primary current of thebeam;

[0106] a secondary electron detector, for detecting a total yield ofelectrons emitted from a surface of the sample in response incidence ofthe beam on the contact openings; and

[0107] a controller, which is adapted a relation between the primarycurrent and the total yield so as to assess a characteristic of the etchprocess.

[0108] There is moreover provided, in accordance with an embodiment ofthe present invention, apparatus for monitoring a process applied to asample having a first layer that is at least partially conductive and asecond layer formed over the first layer, contact openings having beencreated in the second layer by an etch process, the apparatus including:

[0109] a test station, including:

[0110] a particle beam source, which is adapted to direct a beam ofcharged particles to irradiate each of a plurality of the contactopenings that are disposed at different, respective positions over asurface of the sample; and

[0111] a current measuring device, which is adapted to produce an etchindicator signal by measuring, in response to irradiation of each of theplurality of the contact openings by the beam of charged particles, atleast one of a specimen current flowing through the first layer and atotal yield of electrons emitted from a surface of the sample; and

[0112] a controller, which is adapted to store a calibrated thresholdlevel of the etch indicator signal for a given set of properties of theetch process, and to compare the respective etch indicator signalproduced with respect to each of the plurality of the contact openingsto the threshold level so as to assess a characteristic of the etchprocess.

[0113] There is furthermore provided, in accordance with an embodimentof the present invention, apparatus for monitoring a process applied toa sample having a first layer that is at least partially conductive anda second layer formed over the first layer, contact openings having beenformed in the second layer by an etch process, the apparatus including:

[0114] a test station, which includes:

[0115] a particle beam source, which is adapted to direct a beam ofcharged particles to irradiate each of a plurality of the openings thatare disposed at different, respective positions across the sample; and

[0116] a current measuring device, which is adapted to measure at leastone of a specimen current flowing through the first layer and a totalyield of electrons emitted from a surface of the sample in response toirradiation of the contact openings by the beam of charged particles,thus producing an etch indicator signal as a function of the respectivepositions of the plurality of the openings; and

[0117] a controller, which is adapted to evaluate a variation of theetch indicator signal across the sample so as to assess a uniformity ofthe etch process.

[0118] There is also provided, in accordance with an embodiment of thepresent invention, a method for process monitoring of a sample having afirst layer that is at least partially conductive, a second, barrierlayer formed over the first layer, and a third, dielectric layer formedover the second layer, the method including:

[0119] etching contact openings in the third layer in a first etchprocess;

[0120] directing a beam of charged particles to irradiate the contactopenings in a first monitoring step following the first etch process;

[0121] measuring at least one of a specimen current flowing through thefirst layer and a total yield of electrons emitted from a surface of thesample in response to irradiation of the contact openings by the beam ofcharged particles in the first monitoring step, thus producing a secondetch indicator signal;

[0122] evaluating the first etch indicator signal to assess a firstcharacteristic of the first etch process;

[0123] further etching the contact openings from the third layer intothe second layer in a second etch process;

[0124] directing the beam of charged particles to irradiate the contactopenings in a second monitoring step following the second etch process;

[0125] measuring the at least one of the specimen current flowingthrough the first layer and the total yield of the electrons emittedfrom the surface of the sample in response to irradiation of the contactopenings by the beam of charged particles in the second monitoring step,thus producing a second etch indicator signal; and

[0126] evaluating the second etch indicator signal to assess a secondcharacteristic of the second etch process.

[0127] Typically, evaluating the first etch indicator signal includesassessing an integrity of the second layer.

[0128] Additionally or alternatively, evaluating the second etchindicator signal includes verifying that at least some of the contactopenings have been etched through the second layer to the first layer.

[0129] There is additionally provided, in accordance with an embodimentof the present invention, apparatus for process monitoring of a samplehaving a first layer that is at least partially conductive, a second,barrier layer formed over the first layer, and a third, dielectric layerformed over the second layer, the apparatus including:

[0130] an etch station, which is adapted to form contact openings in thethird layer in a first etch process, and subsequently to further etchthe contact openings from the third layer into the second layer in asecond etch process;

[0131] a test station, which includes:

[0132] a particle beam source, which is adapted to direct a beam ofcharged particles to irradiate the contact openings; and

[0133] a current measuring device, which is adapted to measure at leastone of a specimen current flowing through the first layer and a totalyield of electrons emitted from a surface of the sample in response toirradiation of the contact openings by the beam of charged particles,thus producing a first etch indicator signal following the first etchprocess and a second etch indicator signal following the second etchprocess; and

[0134] a controller, which is adapted to evaluate the first etchindicator signal in order to assess a first characteristic of the firstetch process and to evaluate the second etch indicator signal in orderto assess a second characteristic of the second etch process.

[0135] The present invention will be more fully understood from thefollowing detailed description of the embodiments thereof, takentogether with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0136]FIG. 1A is a schematic top view of a semiconductor wafer with atest pattern comprising an array of contact holes formed therein, inaccordance with an embodiment of the present invention;

[0137]FIG. 1B is a schematic top view of another test pattern comprisingsparse and dense arrays of contact holes, in accordance with anembodiment of the present invention;

[0138]FIG. 1C is a schematic top view of a further test patterncomprising arrays of contact holes of different diameters and densities,in accordance with an embodiment of the present invention;

[0139] FIGS. 2A-2E are schematic, sectional illustrations of an area ofa semiconductor wafer, showing a contact hole etched into the waferunder different process conditions;

[0140]FIG. 3A is a block diagram that schematically illustratesapparatus for testing contact hole production, in accordance with anembodiment of the present invention;

[0141]FIG. 3B is a schematic, sectional, detail view of a semiconductorwafer under test, illustrating periodic measurement of specimen current,in accordance with an embodiment of the present invention;

[0142]FIG. 3C is a schematic plot showing waveforms of an AC electronbeam irradiating a semiconductor wafer and specimen current measured asa result of the irradiation, in accordance with an embodiment of thepresent invention;

[0143]FIG. 4 is a schematic, sectional view of the array of contactholes in the test pattern of FIG. 1, taken along a line IV-IV;

[0144]FIG. 5 is a schematic plot of specimen current as a function ofhole size, for the array of contact holes shown in FIG. 4;

[0145] FIGS. 6-8 are schematic plots of specimen current measured as afunction of contact hole position over the surface of a sample, inaccordance with an embodiment of the present invention;

[0146]FIGS. 9A and 9B are schematic plots of specimen current measuredas a function of contact hole position over the surface of a sample,illustrating calibration thresholds used in contact hole monitoring, inaccordance with an embodiment of the present invention;

[0147]FIG. 10 is a schematic top view of a cluster tool that includes acontact hole test station, in accordance with an embodiment of thepresent invention;

[0148]FIG. 11 is a schematic, sectional view of a contact hole on whichan electron beam is incident at a non-normal angle, in accordance withan embodiment of the present invention;

[0149]FIG. 12 is a schematic, pictorial illustration of a biasingelectrode used in conjunction with an electron beam to precharge asurface of a sample, in accordance with an embodiment of the presentinvention; and

[0150]FIG. 13 is a schematic, sectional illustration showingsimultaneous irradiation of a sample by electron and light beams, andmeasurement of the resultant specimen current, in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS System and Method Overview

[0151] Reference is now made to FIG. 1A, which is a schematic top viewof a semiconductor wafer 20 with a test pattern 22 formed thereon, inaccordance with an embodiment of the present invention. The testpattern, comprising an array of contact holes 26, is shown enlarged inan inset. Although only the single test pattern 22 is shown in FIG. 1,multiple test patterns may be distributed over the surface of wafer 20.Other types of test openings and test patterns may also be used, such asthe types described in the above-mentioned U.S. patent application Ser.No. 10/209,087. The test patterns may be located on scribe lines 24between adjacent dice on wafer 20, so as to minimize the loss of usefulspace on the wafer. Additionally or alternatively, the methods ofcontact hole evaluation described hereinbelow may be applied, mutatismutandis, to contact openings formed in functional areas of the dice.

[0152] Holes 26 in test pattern 22 may be graduated in diameter fromlarge to small, as shown in the figure. The size gradation of the holesis particularly useful in assessing the state of an etch process used inprocessing wafer 20, as described below with reference to FIGS. 4 and 5.Typically, the holes are designed to range between 50 nm and 1 μm indiameter and are spaced at least one diameter apart. These dimensionsand spacing of the holes and of the test pattern are cited by way ofexample, however, and other dimensions and spacing may likewise be used(The spacing between holes 26 may also be varied, as described below.)Although FIG. 1A shows a single row of holes 26, the holes may also bearranged in a two-dimensional pattern, and some of the holes may havethe same diameter. The test pattern may also include other types ofcontact openings (not shown), such as trenches or vias.

[0153]FIG. 1B is a schematic top view of another test pattern 21, whichmay be formed on a semiconductor wafer, in accordance with an embodimentof the present invention. Pattern 21 comprises contact holes 26 arrayedin two patterns: a sparse pattern 23, and a dense pattern 25. Typically,holes 26 have the same diameter in both the sparse and dense patterns.As noted above, the etch rate of contact holes is typically lower indense pattern 25 than in sparse pattern 23, due to micro-loadingeffects. The spacing of the contact holes in dense and sparse arrays inthe test structure is typically chosen to represent limiting cases ofactual contact hole spacing for in-die patterns. Therefore, by measuringspecimen current through contact holes in the dense and sparse arrays,it is possible to detect etch problems that may occur within in-diepatterns due to micro-loading.

[0154]FIG. 1C is a schematic top view of still another test pattern 36within the area of a scribe line 24 on a semiconductor wafer, inaccordance with an embodiment of the present invention. Pattern 36combines the principles of the patterns shown above in FIGS. 1A and 1B.Pattern 36 comprises arrays 23 and 25 of sparse and dense contact holes,whose diameters are approximately equal to the critical dimension (CD)of the in-die functional contact holes that are produced on the wafer.In addition, the pattern comprises a dense array 27 and a sparse array29 of contact holes having a smaller diameter than the in-die contactholes; and a dense array 31 and a sparse array 33 of contact holeshaving a larger diameter than the in-die contact holes. An alignmenttarget 35 is typically provided in pattern 36 to facilitate opticalalignment of an inspection system that is used to make specimen currentmeasurements on the pattern, as described hereinbelow. Pattern 36 mayalso include an area that includes no contact holes, for use inestablishing a calibration baseline for the specimen currentmeasurements made on the pattern.

[0155] FIGS. 2A-2E are schematic, sectional illustrations of an area ofa semiconductor wafer, showing formation of contact hole 26 underdifferent process conditions. In a typical application, a non-conductingoxide layer 30 is formed over a silicon substrate layer 28, andphotoresist (not shown in the figure) is deposited on the oxide layer.After photolithographic exposure of the photoresist to define thelocations and dimensions of contact openings in the oxide layer, anetching process is applied to create the contact holes.

[0156] In the exemplary application shown in these figures, hole 26 ismeant to provide a contact to a region 34 of substrate layer 28 thatcontains TiSi₂ for enhanced conductivity. Region 34 may be part of atransistor structure, formed within layer 28 by methods known in theart. Oxide layer 30 typically comprises materials such as undopedsilicon glass (USG), phosphorus silicon glass (PSG), boron phosphorussilicon glass (BPSG), carbon-doped oxide (CDO) or low-k dielectrics. Abarrier layer (not shown in this figure), sometimes referred to as anetch stop layer, which is typically made of silicon nitride, siliconcarbide or a low-K barrier material, such as Applied Materials BLOk™,may be added between the silicon substrate and the dielectric. Thestructure illustrate in these figures, however, is shown solely by wayof example, and holes 26 may likewise be made in and adjacent to otherstructures. Similarly, such contact holes may be used to contactintermediate semiconductor or conductive layers (not shown) formed abovesubstrate 28, rather than contacting the substrate itself directly.

[0157] Holes 26 in test pattern 22 are formed by the same processes ofmaterial deposition, photolithography and etching as are the functionalcircuit features on the wafer that the pattern is intended to test.Within holes 26, substrate layer 28 is exposed to the same extent as itis exposed by etching of contact holes of similar diameter and spacingin functional areas of the wafer. A measurement of the specimen currentgenerated when pattern 22 is irradiated by an electron beam isindicative of the extent to which layer 28 (or an overlyingsemiconductor or conductive layer) is exposed within the holes. Tofacilitate this measurement, a conductive contact pad (not shown in thefigure) may be formed on the underside of wafer 20, below pattern 22.Apparatus and methods used in measuring the specimen current are shownin the figures that follow and are described with reference thereto.

[0158]FIG. 2A shows a perfectly-etched, open hole, i.e., a contact holethat cleanly exposes layer 28 as desired. The remaining figures in thisset show the results of different process problems or defects. In FIG.2B, hole 26 is underetched, typically due to a problem in the etchingprocess or in the uniformity of oxide layer 30, for example.Consequently, the area of layer 28 that is exposed within hole 26 issmaller than it should be. In this case, the specimen current generatedwhen the area of hole 26 is irradiated by an electron beam will besmaller than the current generated in the case of FIG. 2A. When the holeis filled with metal or other conductive material in order to contactlayer 28, the contact resistance may be higher than it should.

[0159] In FIG. 2C, the etching process is too strong or has continuedfor too much time, leading to overetching of hole 26. In this case, thespecimen current will typically be greater than in the case of FIG. 2A.Overetching may have a deleterious effect on region 34 and on otherstructures, and may also lead to deposit of contaminants at the bottomof hole 26.

[0160]FIG. 2D shows a case of severe underetching, in which hole 26stops short of reaching layer 28, typically due to some serious processdefect. For this sort of closed contact hole, the measured specimencurrent will be very low, and the contact resistance when the hole isfilled with metal will be very high.

[0161] Finally, in FIG. 2E, although hole 26 was properly etched, acontaminant 38, such as photoresist residue or fluorocarbon polymer, isdeposited at the bottom of the hole. This contaminant will typicallycause a decrease in the measured specimen current. If the residue is notremoved, it may cause a high contact resistance when hole 26 is filledwith metal. This high contact resistance is a critical process problem,which can generally be detected (using methods known in the art) onlymany process steps later, after the metal layer has been deposited inthe holes usually by electrical testing.

[0162]FIG. 3A is a block diagram that schematically illustrates astation 40 for contact hole inspection, in accordance with an embodimentof the present invention. Station 40 comprises a chamber 42, containinga motion stage 44 on which wafer 20 is placed during inspection. Anelectron gun 46 (or other charged particle source) directs a beam atwafer 20, while an ammeter 48 measures the specimen current generated inthe wafer. The ammeter is typically electrically coupled to the lowerside of wafer 20, in electrical contact with substrate layer 28.Alternatively, the ammeter may be coupled directly to an intermediatesemiconductor or conductive layer in the wafer, assuming that the layerson wafer 20 are suitably configured to enable such coupling. As notedabove, the wafer may include one or more contact pads for use incoupling ammeter 48 to the substrate or intermediate layer.

[0163] The electron beam generated by gun 46 typically has a diameterand energy parameters that can be controlled as required by theapplication. The diameter may be adjusted to cover a single contact holeon the wafer, or expanded to irradiate several holes at once or toprecharge the wafer surface. An adjustment range of 0.5-30 μm in beamdiameter is generally adequate for these purposes. The electron energyof the gun may be variable, typically between about 100 and 5000 eV, soas to cover both positive and negative charging domains of the materialsin wafer 20. (The positive charging domain is the range of electronenergies in which the total yield of secondary and backscatteredelectrons from the surface layer is greater than the primary electronbeam current, while the negative charging domain in the range in whichthe total yield is less than the primary beam current. These phenomena,which are well known in the art, are described in the above-mentionedbook by Yacobi et al. on pages 38-39.) A suitable electron gun for thispurpose, for example, is the EKF 1000 small-spot electron source,produced by Omicron NanoTechnology GmbH (Taunusstein, Germany). This gunis considerably smaller and less expensive than the high-resolutionelectron beam devices used in typical SEM systems. Alternatively,electron guns of other types, as well as other types of particle beams,may be used in station 40.

[0164] The specimen current due to irradiation of contact holes in wafer20 is typically measured in steady state. For this purpose, the area ofthe contact hole to be irradiated is precharged by the beam from gun 46.This precharging may take place as a separate, preliminary stage, beforemaking the specimen current measurements, or it may alternatively becarried out simultaneously with the measurements. The wafer surface maybe negatively precharged, by operating the electron gun at an energy inthe negative charging domain. For photoresist, this condition typicallyholds for all values of the electron beam energy. For SiO₂, a higherbeam energy, preferably above 2 keV, can be used to give negativecharging. Alternatively, a very low-energy beam can be used for negativecharging.

[0165] Further alternatively, a bias electrode 53, which is negativelybiased by a biasing power supply 55, may be used to induce negativecharging by low-energy electrons. This application of the bias electrodeis described below in detail with reference to FIG. 12. As yet a furtheralternative, negative charging of the surface may be achieved byapplying an appropriate electric field bias to the wafer surface, usinga charge control plate as described in U.S. patent application ______{AMAT Docket #0068031, which is assigned to the assignee of the presentpatent application, and whose disclosure is incorporated herein byreference. In any case, negative precharging of the wafer surface causesholes 26 to act as Faraday cups, so that relatively few electrons escapefrom the holes.

[0166] Stage 44 positions wafer 20 so that each of contact holes 26 tobe tested is properly located in turn in the beam of gun 46. Given theminimum diameter of the electron beam, positioning resolution of about±3 μm is generally sufficient unless specific, individual contact holesare to be measured. For simplicity and economy of space, stage 44 maycomprise an R-theta (translation/rotation) stage. Alternatively oradditionally, any other type of motion system with sufficient accuracymay be used for this purpose. For example, the stage may provide X-Ytranslation, or gun 46 may be translated over wafer 20, or the electronbeam itself may be deflected. When test holes or test patterns areprovided on wafer 20 at multiple locations, stage 44 may position thewafer (or the electron gun may be translated or its beam deflected) sothat several of these test holes or patterns are irradiated by theelectron beam in succession. The specimen current is measured at eachhole location, in order to ensure that contact hole uniformity ismaintained over the entire wafer, as described further hereinbelow.Additionally or alternatively, if different test holes or test patternson the wafer are designed to test different sizes or shapes of contactopenings, the specimen current can be measured for each hole size orpattern type.

[0167] During the specimen current measurements, the beam energy of gun46 is typically set to be in the negative charging domain of the topdielectric (background) layer, in order to provide optimal contrastbetween good, open contact holes and those that are closed orunderetched. (As noted above, “open” contact holes are those that whenfilled with conductive material will be electrically conductive with lowresistance; while holes that are closed, underetched or have residueremaining at the bottom may be electrically unconnected or exhibit highresistance when filled with the conductive material.) Typically, lowerbeam energy enhances the sensitivity of the measurement to thin layersof residual dielectric material at the bottom of the contact holes.Optionally, a number of different electron beam energies may be used totest the specimen current at a number of different points on the yieldcurve.

[0168]FIGS. 3B and 3C illustrate an alternative method for measuringspecimen current, in accordance with another embodiment of the presentinvention. If it is not possible to make a good ohmic contact betweenthe semiconductor or conducting layer at the bottom of the contact holesand ammeter 48, the electron beam may be pulsed, and the specimencurrent measured by capacitive coupling. This arrangement is illustratedin FIG. 3B, which is a schematic, sectional view showing a detail of awafer comprising substrate 28 and dielectric 30, with an additionalback-side dielectric layer 63 below the semiconductor substrate. Aphotoresist layer 72 overlying dielectric layer 30 is used in creatingcontact holes 26. An electron beam 61 irradiates the area of contactholes 26. A beam blanking assembly 59 periodically applies a voltageV_(BB) in order to pulse the electron beam on and off. The beam blankingassembly may comprise, for example, a pair of parallel metal plates,between which the electron beam travels before passing through anaperture. When a voltage is applied between the plates, the electronbeam is deflected and does not pass through the aperture to reach thewafer. The resultant AC specimen current is measured using an AC ammeter65, which is capacitively coupled to substrate 28 through dielectriclayer 63.

[0169]FIG. 3C schematically shows the time variation of the electronbeam current, which is tracked by the time variation of the specimencurrent. (Possible smoothing and phase shift of the specimen currentwaveform due to impedance effects are neglected here for the sake ofsimplicity.) An upper specimen current curve 67 illustrates the expectedspecimen current level when contact holes 26 are adequately etched. Alower specimen current curve 69 illustrates the reduction in specimencurrent that occurs due to etch problems, such as the underetching shownin FIG. 3B.

[0170] Alternatively, when a wafer under test has a back-side dielectriclayer, the wafer may be mounted on a chuck with pins that contact theback side of the wafer. A pulsed voltage is applied to the pins in orderto locally penetrate the dielectric layer and establish a good contactwith substrate 28. The resistance between the pins may be measured inorder to determine when sufficient penetration of the dielectric layerhas been achieved.

[0171] Returning now to FIG. 3A, the current of electrons emitted fromwafer 20 may be measured, additionally or alternatively, using asecondary electron detector 49, as is known in the art. As noted above,the specimen current generated in a sample due to irradiation by anelectron beam is equal to the difference between the primary beamcurrent and the total electron yield of the specimen due to secondaryand backscattered electrons. Therefore, it is possible to determine thespecimen current by measuring precisely the primary beam current and thetotal current of secondary and backscattered electrons, withoutmeasuring the specimen current itself directly. This approach typicallyrequires collection of secondary and backscattered electrons high withefficiency—preferably >90%. This high efficiency can be achieved, forexample, using a magnetic immersion lens, which forms at magnetic bottleat the surface of the specimen. Lenses of this sort are described inU.S. Pat. Nos. 4,864,228 and 4,912,405, whose disclosures areincorporated herein by reference. Alternatively, the secondary electroncurrent may be used in conjunction with direct measurement of thespecimen current in order to provide additional information that iscomplementary to the specimen current measurement.

[0172] The positioning and operation of gun 46 and stage 44 arecontrolled by a main controller 50, via a gun control unit 52 and astage control unit 54. Typically, a pre-alignment unit based on alow-resolution optical microscope (OM/PAL) 56 is used by controller 50,via an OM/PAL control unit 58, to locate the test pattern on the waferfor positioning and alignment purposes. Suitable microscopes for thispurpose are made, for example, by Optem (Fairport, N.Y.). Duringoperation, a vacuum is maintained in chamber 42 by a vacuum pump 60,which is also controlled and monitored by controller 50, via a vacuumcontrol unit 62. A robot 64 inserts wafers into chamber 42 and removesthem from the chamber. Controller 50 communicates with the robot via arobot control unit 66. Robot 64 may be used to transfer wafers to andfrom other stations in a cluster tool, as shown below in FIG. 10.

[0173] After positioning stage 44 and firing gun 46 to irradiate one ormore of contact holes 26, controller 50 receives the specimen currentmeasured by ammeter 48. It compares the measured current to benchmarksthat have been established for the expected hole size, materials, etchconditions and other applicable process parameters. Methods fordetermining these benchmarks are described hereinbelow with reference tothe figures that follow. If the controller determines that the measuredcurrent is outside a predetermined tolerance range of a given benchmark,it typically interrupts the production process and notifies a systemoperator via a user workstation 68. The operator evaluates the testresults and then implements whatever corrective action may be necessary.

[0174] The corrective action may include performing further etching, ifthe contact holes are underetched (as shown in FIG. 2B or 2D), orremoving polymer residue that may have been deposited at the bottoms ofthe holes (FIG. 2E). In the latter case, it may be possible to removethe polymer film by high-density electron beam exposure, using electrongun 46. For this purpose, electron beam energy between about 5 and 20keV, with beam current greater than 1 nA, is expected to givesatisfactory results. Thus, station 40 may be used for processcorrection, as well fault detection.

“Early Warning” Test Pattern

[0175]FIG. 4 is a schematic, sectional view of test pattern 22 (FIG. 1),in accordance with an embodiment of the present invention. The testpattern is shown here following completion of an etching process. Asnoted above, the diameters of contact holes 26 in pattern 22 aregraduated from largest (at the left of the figure) to smallest (at theright), ranged above and below the diameter of a nominal hole 70. Thediameters of holes 26 are defined by a photolithographic process appliedto photoresist layer 72, wherein nominal hole 70 is chosen to haveapproximately the same diameter as functional contact holes etched infunctional areas of wafer 20.

[0176] The rate at which the etch process creates a contact holeincreases as the contact hole diameter increases. Therefore, the etchstate of nominal hole 70 should be approximately the same as that offunctional contact holes in wafer 20. As shown in FIG. 4, uponsatisfactory completion of the etch process, the holes of nominaldiameter (i.e., the diameter of hole 70) and larger are etched throughcompletely to substrate 28. Below the nominal diameter, the etching rateis slower, and therefore the depth of the holes decreases withdecreasing hole diameter.

[0177] The situation shown in FIG. 4 is indicative of aproperly-adjusted etch process, in that nominal hole 70 is fully etchedthrough to the substrate, without overetching. There is a safety marginin the process (known as a “process window”), in that the contact holesin pattern 22 that are slightly narrower than the nominal hole are stilletched through to the substrate (so that the nominal hole may beslightly overetched, but not to any deleterious extent). If stillnarrower holes were etched through to the substrate, there would be adanger of overetching the functional holes to which nominal hole 70corresponds. On the other hand, if the holes just slightly narrower thannominal hole 70 were underetched (even if hole 70 still appears to befully etched), there would be a danger of underetching the functionalholes. Thus, monitoring the etching of test pattern 22 can provide anearly warning of process defects, so that prompt corrective action canbe taken. If these incipient defects were allowed to persist, they couldresult in improper etching of functional contact holes in the waferunder test or in other wafers processed subsequently in the same etchingchamber as the current wafer.

[0178]FIG. 5 is a schematic plot of specimen current as a function ofthe inverse of the hole diameter, measured with respect to test pattern22 under two slightly different sets of etch conditions. Each data pointin the plot corresponds to a measurement of specimen current made whileirradiating one of holes 26 with an electron beam. (Typically, the spotsize of the beam is larger than the hole diameter.) An upper curve 80shows the specimen current measured for the set of hole depths shown inFIG. 4. The specimen current decreases gradually in proportion to thehole diameter down to a shoulder value, below which the current dropsmore sharply. This shoulder corresponds to the point at which the holesare no longer fully etched, leaving a highly-resistive dielectric layerat the bottom of the hole, with thickness increasing as hole diameterdecreases. In curve 80, the shoulder occurs several points to the rightof nominal hole 70, indicating that the etch process parameters areproperly adjusted.

[0179] A lower curve 82 shows a change in the measured specimen currentthat may occur when the etch parameters drift from proper adjustment.The shoulder in curve 82 occurs closer to the point of nominal hole 70,although the specimen current measured through the nominal hole stillindicates complete etching. In such a case, controller 50 may alert workstation 68 (FIG. 3) that a process deviation may be occurring, eventhough the etch state of the contact holes in the current wafer is stillsatisfactory. The operator can then correct the etch process before thedeviation becomes severe enough to cause a reduction in the productionyield.

Specimen Current Measurements and Threshold Calibration

[0180]FIG. 6 is a plot of specimen current measured as a function ofposition across two test wafers using, for example, station 40 (FIG. 3),in accordance with an embodiment of the present invention. The verticalaxis, representing the measured current in this figure and in FIGS. 7and 8, is logically reversed, i.e., the measured current is negative,and the magnitude of the current increases from the top to the bottom ofthe plot. The measurements were made on contact holes at differentlocations along a diameter of the wafer, from one side of the wafer tothe other. The contact holes were etched down to silicon substrate 28,without intervention of a nitride stop layer below oxide 30. The etchstates of the holes were verified after measurement by cross-sectionalimaging of the contact holes.

[0181] A first curve 90 was measured on a properly-etched wafer, inwhich the contact holes were etched for approximately 30% longer thannominal. Based on these measurements, it is possible to define athreshold 92, corresponding to satisfactory, normal etching of thewafer. The variation of the specimen current over the surface of thewafer may be used to define control limits, over which the specimencurrent is permitted to vary and still be considered within theacceptable range.

[0182] A second curve 94 was measured on a wafer of the same type ascurve 90, which was etched using non-uniform process parameters. As aconsequence, the contact holes in the central area of the wafer wereunderetched, resulting in low measured specimen current, while those onthe periphery of the wafer were etched properly. Both these conditionsare detected by station 40. A non-uniform profile, such as that of curve94, is typically sufficient to indicate that a process problem existsand to notify a system operator or automatically stop processing wafers,even if the results are not above a specific absolute threshold.

[0183] The specimen current measurements shown in FIG. 6, as well asthose shown in the figures that follow, were compared to cross-sectionalimages of the wafers that were tested, and a good correlation was foundbetween the specimen current levels and the actual etch states of thecontact holes.

[0184]FIG. 7 is a plot of specimen current measured as a function ofposition of contact holes distributed across another test wafer, inaccordance with an embodiment of the present invention. In this case,the wafer included a nitride stop layer below the oxide that was etched.The low specimen current measured upon irradiation of the central pointsin the curve is indicative of underetching of the holes at these points.Toward the edges of the wafer, the holes were fully etched, down to thenitride layer. Because of the relatively high conductivity of siliconnitride, relative to silicon (which may have been enhanced by theelectron beam irradiation), the specimen current flowing through theseholes is considerably greater than that shown in FIG. 6.

[0185]FIG. 8 is a plot of specimen current measured as a function ofposition of contact holes distributed across yet another test wafer, inaccordance with an embodiment of the present invention. This figureillustrates the capability of station 40 to detect residues in etchingof the nitride etch stop layer (which is typically performed as aseparate process step, to remove the barrier layer from the bottom ofcontact holes, after first etching the hole through the overlyingoxide). The nitride layer was etched out of the contact holes on oneside of the wafer, shown to the left in the plot of FIG. 8, but was leftintact on the other side. It can be seen from this figure that themethods of the present invention may be used to monitor not only thestate of an oxide etching process, but also other etching processes,including nitride etching.

[0186]FIGS. 7 and 8 thus demonstrate that the methods of the presentinvention may be used to monitor etching of dual dielectric layers(upper dielectric with stop layer below). Etch stop layers are now usedin many applications, particularly high aspect ratio contact and viaprocesses in devices such as DRAM. The dual dielectric layers aretypically etched in two different, successive etch steps, one for eachlayer. It is important that the first etch step, illustrated by FIG. 7,reaches but does not punch through the stop layer. Punch-through mayoccur, for example, due to low selectivity in the first etch step or touse of a very thin stop layer. The punch-through would be evidenced byan abnormally large value of the specimen current upon conclusion of thefirst etch step, while underetching in the first etch step gives lowspecimen current, as shown in FIG. 7. The second etch step, in which thecontact holes are etched through the stop layer may be monitored insimilar fashion, but with different threshold levels to indicate properetching.

[0187]FIG. 9A shows schematic plots 103, 104, 105 and 106 of specimencurrent (absolute values), measured as a function of contact holeposition for a number of different samples. The measured currents areused in calibrating absolute process control limits, in accordance withan embodiment of the present invention. Plots 103-106 are measured usingtest structures and measurement methods such as those described above.The measurements of specimen current from specific contact holes arecompared to cross-sectional images of the same contact holes. Some orall of the dies used in the specimen current measurements may besectioned for this purpose. These measurements are then used inestablishing an upper excursion limit 100 and a lower excursion limit102, marking the bounds of measured specimen current values 108 thatcorrespond to acceptable contact holes.

[0188] Plot 106 illustrates specimen current measurement values 109 thatwere made on an underetched wafer. In this case, only the contact holeson the wafer periphery were adequately etched. (Whether the peripheralholes are etched differently from those nearer the center of the waferdepends on factors such as the etcher type, process recipe andmaterials, inter alia.)

[0189] Plot 105 was taken from a slightly overetched wafer, anddemonstrates properly-etched contact holes over the entire waferdiameter. Proper etching of the contact holes is verified bycross-sectional imaging. Plot 105 can be used to establish lowerexcursion limit 102, based on the minimal absolute value of the specimencurrent on this plot, taking the estimated measurement error(illustrated by the error bars in the figure) into consideration. If thecross-sectional images show any of the holes on this wafer to beunderetched, on the other hand, another wafer may be etched, using alonger etch time, and may be tested in like manner to establish thelower excursion limit.

[0190] Plot 104 illustrates specimen current measurements made on awafer etched according to an optimized method, in accordance with anembodiment of the present invention, which typically corresponds toextending the etching time by 10-30% compared to that used in generatingplot 105. The minimal specimen current measurement on plot 106 is usedin determining a lower control limit (LCL) 101, again taking intoaccount the estimated measurement error. When the calibration boundsdetermined by the present method are used in monitoring productionwafers, and the measured specimen current drops below LCL 101, an earlywarning signal may be issued to warn of possible process drift.Typically, the specimen current measurements from a number of wafers maybe analyzed statistically in order to set LCL 101, so as to account fornormal etch process variations.

[0191] Plot 103 shows specimen current measurements taken from astrongly-overetched wafer. In this case, measurement values 107 areindicative of punch-through of a stop layer below the main dielectriclayer being etched, as described above. The punch-through is verified bycross-sectional images. Upper excursion limit 100 is set to correspondto the maximal specimen current value below the error bounds ofmeasurements 107.

[0192]FIG. 9B schematically illustrates specimen current measurementsused in calibrating relative control limits, in accordance with anembodiment of the present invention. The inventors have found thatunderetched wafers tend to exhibit very high non-uniformity of specimencurrent measurements taken across the wafer diameter. As shown in FIG.9B, the non-uniformity in a plot 112 taken on an underetched wafer mayreach 100%, as indicated by a maximum current value 113 and a minimumcurrent value 114 reached by this plot. By contrast, plots 110 and 111show that for properly-etched wafers, the non-uniformity is typically nomore than 10-15%. Non-uniformity is also significant in overetchedwafers in which punch-through has occurred.

[0193] The relative control limit for non-uniformity is thus a singlevalue, indicating the maximal permitted variation among specimen currentmeasurements taken over the diameter of a wafer. It can be determinedfrom plot 111, for example, which shows the measurements made on aslightly-overetched wafer, as verified by cross-sectional imaging. Therelative control limit is typically applied in subsequent measurementson production wafers as an average non-uniformity value (including errorbars). Excursions of the average non-uniformity above the relativecontrol limit are considered to indicate underetching or punch-throughin the contact holes on the wafer under test. The use of such a relativecontrol limit is advantageous in that it provides fast, reliable processmonitoring, which is insensitive to variations in the specimen currentdue to drift in the primary electron beam current.

[0194] Alternatively or additionally, the primary electron beam currentmay be monitored, and the ratio of the specimen current to the primarybeam current may be used as an etch quality indicator.

[0195]FIG. 10 is a schematic top view of an etch process cluster tool120 in which test station 40 is integrated, in accordance with anembodiment of the present invention. This integration is made possibleby the small size and simplicity of the components of station 40. Robot64 receives wafer 20 through a load lock 21, after photoresist has beendeposited over oxide layer 30 and has then been exposed byphotolithography to form circuit features including contacts and/orvias, with a suitable test pattern, such as pattern 22. Since theinterior of tool 120 is evacuated, robot 64 is able to transfer wafer 20from chamber to chamber without exposing the wafer to ambient air.Typically, the wafer is inserted in an etching station 124. At thisstage, holes 26 are formed through layer 30, preferably by a reactiveion etching process. The foregoing steps are known in the art and aredescribed here solely by way of illustration. Other arrangements of thestations in tool 120 may similarly be used.

[0196] After etching of holes 26 in wafer 20, the wafer is passed totest station 40. At this point, the wafer (except for the etched holes)is still covered by a layer of exposed photoresist. In station 40, thespecimen current from wafer 20 is measured at one or more pre-definedpoints, either in product dies or on test structures or both. Theresults are evaluated, as described above, by a controller 128 (whichmay incorporate the functions of controller 50, shown in FIG. 3).Typically, the controller evaluates the specimen current for multipleholes distributed across the wafer, as shown in the preceding figures,and compares the measured values to both absolute and relativethresholds for the process in question. If the specimen current for allholes measured is within the tolerance range defined by the thresholds,the contact holes in the wafer are deemed to be acceptable. Robot 64then moves wafer 20 into a plasma ashing station 126 for removal of theremaining photoresist, and to a cleaning station 122. If desired, thecontact hole test in chamber 40 may be repeated after the ashing stage.

[0197] On the other hand, if the specimen current measured in station 40is too low, indicating that the holes have been underetched, robot 64may be instructed (automatically or manually) to return the wafer toetching station 74 for further etching, to be followed by re-test instation 40. Under these circumstances, controller 128 typically issuesan alarm to workstation 68, as well, indicating to the operator that anadjustment of process parameters may be needed. Alternatively,controller 128 may autonomously adjust certain process parameters(increasing or decreasing the etch duration in etching station 124, forexample), in response to deviations of the specimen current from idealbehavior.

Contact Hole Measurements Using an Angled E-Beam

[0198]FIG. 11 is a schematic, sectional illustration showing angledirradiation of contact hole 26 by an electron beam 130, in accordancewith an embodiment of the present invention. The tilt angle of beam 130is preferably chosen so that a majority of primary beam electrons do notstrike the bottom of the contact hole. This condition can be achievedwhen the following geometrical condition is satisfied:

α>arctan(1/AR),

[0199] wherein α is the tilt angle, and AR is the aspect ratio (ratio ofdepth to diameter) of the contact hole.

[0200] As a result of the tilt angle, the primary electrons hit the sidewall of the hole 26 rather than the bottom. The electron bombardmentcauses emission of secondary electrons with low energy (typically <50eV). The low-energy secondary electrons can be forced down to the holebottom, rather than moving out of the hole, by negatively precharging asurface 132 of the wafer around the hole. If the contact hole is etchedproperly (with no residue left at the bottom), the low-energy electronflow will pass through substrate 28 and will thus be measured as aspecimen current by ammeter 48. If a thin residue (even tens ofAngstroms thick), such as under etched dielectric or contaminant 38, isleft after the etch, the specimen current will be much lower, due to lowpenetration depth of the low-energy secondary electrons.

[0201] Similar results may be achieved using a very low energy(preferably 50-500 eV) primary electron beam at normal incidence. Thelower energy in either case reduces the interaction volume at thesurface of the bottom of the hole and thus substantially increases thesensitivity of the specimen current to thin layers.

[0202] By contrast, when electron beam 130 operates at higher energy andis not angled, the energetic primary electrons reach the bottom of hole26. In this case, the interaction volume is larger, and the electronsthus pass easily through contaminant 38. Therefore, holes withparticularly thin residues cannot be readily distinguished from holesthat have been etched satisfactorily. Angling electron beam 130 thusprovides an electron energy transformation inside hole 26, whichsubstantially enhances sensitivity of the specimen current measurementto very thin residues at the hole bottom. This method is particularlyuseful in detecting fluorocarbon polymer residue, organic photoresistresidue, and extremely thin oxide, nitride or other dielectric residue(including low-k dielectric with a corresponding stop layer). It can beperformed by station 40 in-line, providing closed-loop monitoring ofreactive ion (plasma) etching, photoresist ashing, and wet polymercleaning steps.

[0203] Tilting of electron beam 130 may be achieved by either mechanicalmeans or by the use of electron optics to control the beam, or by acombination of both techniques. Some CD SEM systems, such as the AppliedMaterials NanoSEM3D, provide this sort of beam tilt capability.

Negative Precharging Using a Bias Electrode

[0204] As noted above, it is desirable in specimen current measurementsfor contact hole monitoring to negatively precharge surface 132 of wafer20. In order to induce a negative precharge on oxide layer 30 usingprimary electron beam 130, it is typically necessary to work at a beamenergy of several keV (up to 5 keV depending on the dielectric type).Such high energy electrons, however, can damage silicide layers and gateoxides in the wafer, which may lead to semiconductor device degradation,failure or yield loss.

[0205]FIG. 12 is a schematic, pictorial illustration, showing how biaselectrode 53 may be used to alleviate this problem, in accordance withan embodiment of the present invention. In this figure, electrode isshown as a ring, with an aperture for beam 130. Alternatively, electrode53 may comprise a fine grid, for example, or may be produced in otherforms, as will be apparent to those skilled in the art. The use of abiased filter mesh of this sort—albeit for other purposes—is describedin European Patent Application EP 0 892 275 A2, whose disclosure isincorporated herein by reference. It is even possible to adapt thebottom electrode (i.e., the electrode next to wafer 20) of a SEMimmersion lens to serve this purpose.

[0206] Electrode 53 is negatively biased by power supply 55. Thenegative bias repels secondary electrons 134 that are emitted fromsurface 132 due to incidence of beam 130, without substantiallyinfluencing the primary electron energy of beam 130. By repelling thelow-energy secondary electrons back to surface 132, electrode 53creating a negative net charge on the dielectric surface. In otherwords, electrode 53 causes the total electron charge leaving the surfaceto be less than the charge acquired by the surface due to the primaryelectron irradiation. The inventors have found that using a 1 keVprimary electron beam with a −50 V bias on electrode 53 providessatisfactory precharging of surface 132. Electrode 53 was about 1 cm indiameter and was placed about 1.5 mm from surface 132. The electrode hada central aperture about 1 mm in diameter through which beam 130 passed.This method of precharging may be used advantageously in conjunctionwith the angled beam irradiation method shown in FIG. 11.

Combined Optical and E-Beam Excitation

[0207]FIG. 13 schematically illustrates an active transistor structure140, which is irradiated simultaneously by e-beam 130 and by an opticalbeam 162 produced by a light source 160, in accordance with anotherembodiment of the present invention. This embodiment is directedparticularly to etch quality assessment of functional contact holes,used in producing microelectronic devices on wafer 20.

[0208] Structure 140 is a typical CMOS structure, which includes an NMOStransistor 142 and a PMOS transistor 144. As is known in the art,transistor 142 is situated in a P-well 146, containing N-typesource/drain 148 and a gate 150, while transistor 144 is situated in anN-well 152 with P-type source/drain 154 and its own gate 150. If contactholes 26 are irradiated by electron beam 130 alone, the P-N-P structureof PMOS transistor 144 will present a very high resistance to anyspecimen current that is generated, regardless of external bias.Therefore, it becomes difficult to make accurate measurements of thequality of contact holes 26 based on the specimen current.

[0209] To solve this problem, light source 160 irradiates structure 140with beam 162, which typically comprises visible, near infrared orultraviolet light. The photon energy of beam 162 is chosen so thatabsorption of the beam causes electron-hole pairs to be generated withinthe P-N junctions of transistors 142 and 144, as well as in P/N wellsand the bulk silicon of substrate 28. In silicon, for example, thefrequency of beam 162 is typically chosen to give photon energy in thevicinity of the bandgap (about 1.12 eV) or above. The presence of thephotoelectrons substantially increases the conductivity of PMOStransistor 144, so that the specimen current upon irradiation byelectron beam 130 can be readily measured. The photoelectrons may alsoenhance the measurement of specimen current from NMOS transistor 142, ifthe natural conductivity due to negatively-charged dielectric 130 isinsufficient.

[0210] Other applications of combined electromagnetic and chargedparticle irradiation in semiconductor wafer inspection will be apparentto those skilled in the art. For example, when electromagnetic energy isapplied during SEM imaging, the contrast properties of circuit featuresmay be altered, thereby providing additional image information thatwould not otherwise be present.

[0211] Although the embodiments described hereinabove are directedparticularly to contact hole monitoring, the principles of the presentinvention may also be applied to other quality control tasks, such asmeasurement and monitoring of other feature dimensions (particularlycritical dimensions) in the semiconductor wafer fabrication process. Themethods of the present invention provide an indication both of the widthof such features and of the thickness of layers making up the features.These methods can be adapted for use not only before metal deposition,as in the embodiments described above, but also after metal depositionto inspect contacts, interconnects and metal lines for disconnects,short circuits and other defects.

[0212] It will be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsubcombinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

1. A method for process monitoring, comprising: receiving a samplehaving a first layer that is at least partially conductive and a secondlayer formed over the first layer, following production of contactopenings in the second layer by an etch process, the contact openingscomprising a plurality of test openings having different, respectivetransverse dimensions; directing a beam of charged particles toirradiate the test openings; measuring, in response to the beam, atleast one of a specimen current flowing through the first layer and atotal yield of electrons emitted from a surface of the sample, thusproducing an etch indicator signal; and analyzing the etch indicatorsignal as a function of the transverse dimensions of the test openingsso as to assess a characteristic of the etch process.
 2. The methodaccording to claim 1, wherein analyzing the etch indicator signalcomprises assessing a residual thickness of the dielectric layer at abottom of the test openings as a function of the transverse dimensions.3. The method according to claim 2, wherein the test openings comprise afirst opening having a first transverse dimension, and at least a secondopening having a second transverse dimension that is less than the firsttransverse dimension, and the method further comprises controlling theetch process, in response to the etch indicator signal, so that thefirst opening is sufficiently deep to reach the first layer, while atleast the second opening is not sufficiently deep to reach the firstlayer.
 4. The method according to claim 3, wherein the test openingsfurther comprise a third opening, having a third transverse dimensionintermediate the first and second transverse dimensions, and whereinanalyzing the etch indicator signal comprises detecting a potentialprocess defect when the etch indicator signal indicates that the thirdopening is not sufficiently deep to reach the first layer.
 5. The methodaccording to claim 2, wherein the sample has a barrier layer formedbetween the first and second layers, and wherein assessing the residualthickness comprises analyzing the etch indicator signal after etchingthe second layer in order to assess an integrity of the barrier layer,and then analyzing the etch indicator signal after etching the barrierlayer.
 6. The method according to claim 1, wherein analyzing the etchindicator signal comprises assessing a critical dimension of a bottom ofthe test openings as a function of the transverse dimensions.
 7. Themethod according to claim 1, wherein analyzing the etch indicator signalcomprises measuring a beam current of the beam of charged particles, andanalyzing a ratio of the etch indicator signal to the beam current. 8.The method according to claim 1, wherein measuring at least one of thespecimen current and the total yield of the electrons comprisesmeasuring the total yield of the electrons emitted from the surface ofthe sample and further comprises measuring a primary current of thebeam, and taking a difference between the primary current and the totalyield to determine the etch indicator signal.
 9. The method according toclaim 1, wherein the plurality of test openings comprises multiplegroups of the test openings in respective test areas, which aredistributed in different locations across the sample, and whereindirecting the beam comprises positioning at least one of the beam andthe sample so as to irradiate each of at least two of the test areas inturn.
 10. The method according to claim 9, wherein analyzing the etchindicator signal comprises evaluating a variation of the etch indicatorsignal across the sample so as to assess a uniformity of the etchprocess.
 11. The method according to claim 1, wherein the plurality oftest openings comprise at least first and second arrays of testopenings, characterized by different, respective first and secondspacings between the test openings in the arrays.
 12. The methodaccording to claim 1, wherein directing the beam comprises irradiatingthe test openings along a beam axis that deviates substantially in anglefrom a normal to a surface of the sample.
 13. The method according toclaim 12, wherein the test openings have side walls and a bottom, andwherein irradiating the test openings comprises angling the beam so thatmore of the charged particles strike the side walls than strike thebottom.
 14. The method according to claim 1, wherein directing the beamcomprises operating the beam so as to precharge a surface of the samplein proximity to the test openings, so as to facilitate measurement ofthe specimen current.
 15. The method according to claim 14, whereinoperating the beam so as to precharge the surface causes electrons to beemitted from the surface, and comprising creating an electric field in avicinity of the surface so as to cause at least a portion of the emittedelectrons to return to the surface, thereby generating a negativeprecharge at the surface.
 16. The method according to claim 1, whereinthe sample comprises a semiconductor wafer, and the contact openingscomprise at least one of contact holes, trenches and vias.
 17. Themethod according to claim 1, wherein receiving the sample comprisesreceiving the sample with a photoresist layer overlying the secondlayer, the photoresist layer having been used in etching the contactopenings, and wherein analyzing the etch indicator signal comprisesmonitoring the etch indicator signal while irradiating the test area,prior to removing the photoresist layer.
 18. The method according toclaim 17, and comprising, if the etch indicator signal indicates that aresidual thickness of the second layer at a bottom of one or more of thetest openings is greater than a predetermined limit, further etching thesecond layer using the photoresist layer so as to increase the depth.19. The method according to claim 1, wherein the sample comprises asemiconductor wafer, and wherein at least some of the contact openingsnot comprised in the plurality of test openings belong to multiplemicroelectronic circuits on the wafer, wherein the circuits areseparated by scribe lines, and the test openings are located on one ofthe scribe lines.
 20. The method according to claim 1, wherein analyzingthe etch indicator signal comprises detecting a residue within thecontact openings, and comprising irradiating the sample with the beam ofcharged particles so as to remove the residue.
 21. The method accordingto claim 1, wherein directing the beam comprises directing a pulsed beamof the charged particles to irradiate the test openings, and whereinmeasuring at least one of the specimen current and the total yield ofelectrons comprises measuring a time variation of the specimen currentby capacitive coupling to the sample.
 22. A method for processmonitoring, comprising: receiving a sample having a first layer that isat least partially conductive and a second layer formed over the firstlayer, following production of contact openings in the second layer byan etch process, the contact openings comprising at least first andsecond arrays of test openings, characterized by different, respectivefirst and second spacings between the test openings in the first andsecond arrays; directing a beam of charged particles to irradiate thearrays of test openings; measuring, in response to the beam, at leastone of a specimen current flowing through the first layer and a totalyield of electrons emitted from a surface of the sample, thus producingan etch indicator signal; and analyzing the etch indicator signal as afunction of the spacings of the arrays of the test openings so as toassess a characteristic of the etch process.
 23. The method according toclaim 22, wherein analyzing the etch indicator signal comprisesassessing a residual thickness of the dielectric layer at a bottom ofthe test openings as a function of the spacings.
 24. The methodaccording to claim 23, wherein the first spacing is substantiallygreater than the second spacing, and the method comprises controllingthe etch process, in response to the etch indicator signal, so that thetest openings in the first array are sufficiently deep to reach thefirst layer, while the test openings in the second array are notsufficiently deep to reach the first layer.
 25. The method according toclaim 23, wherein the sample has a barrier layer formed between thefirst and second layers, and wherein assessing the residual thicknesscomprises analyzing the etch indicator signal after etching the secondlayer in order to assess an integrity of the barrier layer, and thenanalyzing the etch indicator signal after etching the barrier layer. 26.The method according to claim 22, wherein analyzing the etch indicatorsignal comprises assessing a critical dimension of a bottom of the testopenings as a function of the transverse dimensions.
 27. The methodaccording to claim 22, wherein analyzing the etch indicator signalcomprises measuring a beam current of the beam of charged particles, andanalyzing a ratio of the etch indicator signal to the beam current. 28.The method according to claim 22, wherein measuring at least one of thespecimen current and the total yield of the electrons comprisesmeasuring the total yield of the electrons emitted from the surface ofthe sample and further comprises measuring a primary current of thebeam, and taking a difference between the primary current and the totalyield to determine the etch indicator signal.
 29. The method accordingto claim 22, wherein the test openings comprise multiple arrays of thetest openings having different, respective spacings in a plurality oftest areas, which are distributed in different locations across thesample, and wherein directing the beam comprises positioning at leastone of the beam and the sample so as to irradiate each of at least twoof the test areas in turn.
 30. The method according to claim 29, whereinanalyzing the etch indicator signal comprises evaluating a variation ofthe etch indicator signal across the sample so as to assess a uniformityof the etch process.
 31. The method according to claim 22, whereindirecting the beam comprises irradiating the test openings along a beamaxis that deviates substantially in angle from a normal to a surface ofthe sample.
 32. The method according to claim 31, wherein the testopenings have side walls and a bottom, and wherein irradiating the testopenings comprises angling the beam so that more of the chargedparticles strike the side walls than strike the bottom.
 33. The methodaccording to claim 22, wherein directing the beam comprises operatingthe beam so as to precharge a surface of the sample in proximity thetest openings, so as to facilitate measurement of the specimen current.34. The method according to claim 33, wherein operating the beam so asto precharge the surface causes electrons to be emitted from thesurface, and comprising creating an electric field in a vicinity of thesurface so as to cause at least a portion of the emitted electrons toreturn to the surface, thereby generating a negative precharge at thesurface.
 35. The method according to claim 22, wherein the samplecomprises a semiconductor wafer, and the contact openings comprise atleast one of contact holes, trenches and vias.
 36. The method accordingto claim 22, wherein receiving the sample comprises receiving the samplewith a photoresist layer overlying the second layer, the photoresistlayer having been used in etching the contact openings, and whereinanalyzing the etch indicator signal comprises monitoring the etchindicator signal while irradiating the test area, prior to removing thephotoresist layer.
 37. The method according to claim 36, and comprising,if the etch indicator signal indicates that the residual thickness ofthe second layer at the bottom of one or more of the test openings isless than a predetermined limit, further etching the second layer usingthe photoresist layer so as to increase the depth.
 38. The methodaccording to claim 22, wherein the sample comprises a semiconductorwafer, and wherein at least some of the contact openings not comprisedin the plurality of test openings belong to multiple microelectroniccircuits on the wafer, wherein the circuits are separated by scribelines, and the test openings are located on one of the scribe lines. 39.The method according to claim 22, wherein analyzing the etch indicatorsignal comprises detecting a residue within the contact openings, andcomprising irradiating the sample with the beam of charged particles soas to remove the residue.
 40. The method according to claim 22, whereindirecting the beam comprises directing a pulsed beam of the chargedparticles to irradiate the test openings, and wherein measuring at leastone of the specimen current and the total yield of electrons comprisesmeasuring a time variation of the specimen current by capacitivecoupling to the sample.
 41. A method for monitoring a process carriedout on a sample, the method comprising: directing a beam of chargedparticles to irradiate the sample along a beam axis that deviatessubstantially in angle from a normal to a surface of the sample;measuring, in response to incidence of the beam on the sample, aspecimen current flowing through the sample; and analyzing the specimencurrent so as to assess a characteristic of the process.
 42. The methodaccording to claim 41, wherein the sample has a first layer that is atleast partially conductive and a second layer formed over the firstlayer, and wherein the process comprises an etch process, which isapplied to the sample so as to produce contact openings in the secondlayer, and wherein directing the beam comprises irradiating the contactopenings, and wherein analyzing the specimen current comprises assessingthe etch process.
 43. The method according to claim 42, wherein some ofthe contact holes are characterized by a tilt relative to the normal tothe surface, and wherein directing the beam comprises angling the beamso as to compensate for the tilt.
 44. The method according to claim 42,wherein the contact openings have side walls and a bottom, and whereindirecting the beam comprises angling the beam so that more of thecharged particles strike the side walls than strike the bottom.
 45. Themethod according to claim 44, wherein the contact openings arecharacterized by an aspect ratio, and wherein directing the beamcomprises aligning the beam axis at an angle that deviates from thenormal to the surface by at least an arctangent of an inverse of theaspect ratio.
 46. The method according to claim 42, wherein analyzingthe specimen current comprises assessing whether a contaminant residueis present within the contact openings.
 47. The method according toclaim 46, and comprising irradiating the sample with the beam of chargedparticles along the normal to the surface so as to remove the residue.48. The method according to claim 42, wherein directing the beamcomprises operating the beam so as to negatively precharge the surfaceof the sample in proximity the contact openings, so as to facilitatemeasurement of the specimen current.
 49. The method according to claim48, wherein operating the beam so as to precharge the surface causeselectrons to be emitted from the surface, and comprising creating anelectric field in a vicinity of the surface, so as to cause at least aportion of the emitted electrons to return to the surface, therebynegatively precharging the surface.
 50. The method according to claim42, wherein the sample comprises a semiconductor wafer, and wherein thecontact openings comprise at least one of contact holes, trenches andvias.
 51. The method according to claim 42, wherein the sample has abarrier layer formed between the first and second layers, and whereinassessing the etch process comprises analyzing the specimen currentafter etching the second layer in order to assess an integrity of thebarrier layer, and then analyzing the specimen current after etching thebarrier layer.
 52. The method according to claim 41, wherein analyzingthe specimen current comprises measuring a beam current of the beam ofcharged particles, and analyzing a ratio of the specimen current to thebeam current.
 53. The method according to claim 41, wherein directingthe beam comprises directing a pulsed beam of the charged particles toirradiate the sample, and wherein measuring the specimen currentcomprises measuring a time variation of the specimen current bycapacitive coupling to the sample.
 54. A method for process monitoring,comprising: directing a beam of charged particles to irradiate a surfaceof a sample, whereby electrons are emitted from the surface; applying anelectric field in a vicinity of the surface, so as to cause at least aportion of the emitted electrons to return to the surface, therebygenerating a negative precharge at the surface; and receiving a signalproduced by the sample in response to the beam and the negativeprecharge.
 55. The method according to claim 54, wherein the sample hasa first layer that is at least partially conductive and a second layerformed over the first layer, and wherein the negative precharge isformed on the surface of the second layer.
 56. The method according toclaim 55, wherein the second layer comprises a dielectric material. 57.The method according to claim 54, wherein receiving the signal comprisesmeasuring at least one of a specimen current flowing through the sampleand a total yield of electrons emitted from the surface of the sample.58. The method according to claim 57, wherein the sample has a firstlayer that is at least partially conductive and a second layer formedover the first layer, with contact openings formed in the second layerby an etch process, and wherein receiving the signal comprises analyzingthe signal so as to assess a characteristic of the etch process.
 59. Themethod according to claim 58, wherein the sample comprises asemiconductor wafer, and wherein the contact openings comprise at leastone of contact holes, trenches and vias.
 60. The method according toclaim 58, wherein the sample has a barrier layer formed between thefirst and second layers, and wherein analyzing the signal comprisesanalyzing the signal after etching the second layer in order to assessan integrity of the barrier layer, and then analyzing the signal afteretching the barrier layer.
 61. The method according to claim 54, whereindirecting the beam comprises operating the beam during a precharginginterval so as to generate the negative precharge at the surface, andthen operating the beam after the precharging interval so as to generatethe signal.
 62. The method according to claim 61, wherein operating thebeam during the precharging interval comprises setting the beam sourceso that electrons have an energy in a positive charging domain of thesurface of the sample.
 63. A method for testing a semiconductor device,comprising: irradiating a junction in the semiconductor device with afirst beam comprising electromagnetic radiation; irradiating the devicewith a second beam comprising charged particles, so that at least someof the charged particles are incident on the junction substantiallysimultaneously with the electromagnetic radiation; and measuring, inresponse to incidence of the first and second beams on the junction, aproperty of the device.
 64. The method according to claim 63, whereinmeasuring the property comprises forming an electronic image of thedevice.
 65. The method according to claim 63, wherein the junctioncomprises a semiconductor material, and wherein irradiating the junctionwith the first beam comprises irradiating the junction with photonshaving an energy greater than or equal to a bandgap of the semiconductormaterial.
 66. The method according to claim 63, wherein the junctioncomprises a P-N junction.
 67. The method according to claim 63, whereinmeasuring the property comprises measuring a current flowing through thedevice.
 68. The method according to claim 67, wherein a dielectric layeris formed over the junction, and a contact hole is formed through thedielectric layer in order to contact the junction, and whereinirradiating the junction with the first and second beams comprisesirradiating an interior of the contact hole, and wherein measuring thecurrent comprises assessing a characteristic of the contact hole basedon the current.
 69. The method according to claim 68, wherein assessingthe characteristic comprises assessing a suitability of the contact holeto make a conductive electrical contact with the junction.
 70. A methodfor process monitoring, comprising: receiving a sample having a firstlayer that is at least partially conductive and a second layer formedover the first layer, following production of contact openings in thesecond layer by an etch process; directing a beam of charged particlesto irradiate one or more of the contact openings; measuring a primarycurrent of the beam and a total yield of electrons emitted from asurface of the sample in response incidence of the beam on the contactopenings; and analyzing a relation between the primary current and thetotal yield of the electrons so as to assess a characteristic of theetch process.
 71. The method according to claim 70, wherein analyzingthe relation comprises analyzing a difference between the primarycurrent and the total yield.
 72. The method according to claim 70,wherein analyzing the relation comprises analyzing a ratio between theprimary current and the total yield.
 73. The method according to claim70, wherein directing the beam comprises irradiating multiple contactopenings, which are distributed in different locations across thesample, and wherein analyzing the relation comprises evaluating avariation of the relation across the sample so as to assess a uniformityof the etch process.
 74. The method according to claim 70, whereindirecting the beam comprises precharging a surface of the sample inproximity to the one or more of the contact openings, and whereinmeasuring the primary current and the total yield comprises measuringthe total yield of the electrons emitted from the precharged surface.75. The method according to claim 70, wherein the sample comprises asemiconductor wafer, and wherein the contact openings comprise at leastone of contact holes, trenches and vias.
 76. The method according toclaim 70, wherein the sample has a barrier layer formed between thefirst and second layers, and wherein analyzing the relation comprisesanalyzing the relation after etching the second layer in order to assessan integrity of the barrier layer, and then analyzing the relation afteretching the barrier layer.
 77. A method for process monitoring of asample having a first layer that is at least partially conductive and asecond layer formed over the first layer, wherein contact openings areformed in the second layer by an etch process, the method comprising:determining, for a given set of characteristics of the contact openings,a threshold level of an etch indicator signal, which is produced bymeasuring at least one of a specimen current flowing through the firstlayer and a total yield of electrons emitted from a surface of thesample in response to irradiation of the contact openings by a beam ofcharged particles; directing the beam of charged particles to irradiateeach of a plurality of the contact openings that have the given set ofcharacteristics and are disposed at different, respective positions overa surface of the sample; determining, in response to the beam, the etchindicator signal produced at each of the respective positions of theplurality of the contact openings; and comparing the etch indicatorsignal produced at the respective positions to the threshold level so asto assess a characteristic of the etch process.
 78. The method accordingto claim 77, wherein comparing the etch indicator signal comprisesdetermining, if an absolute magnitude of the specimen current fallsbelow the threshold level by more than a predetermined margin, that atleast some of the contact openings are underetched.
 79. The methodaccording to claim 77, wherein determining the threshold level comprisesfinding the level of the etch indicator signal that corresponds toetching of the contact openings through the second layer to expose thefirst layer within the opening.
 80. The method according to claim 79,wherein finding the level comprises calibrating the threshold level in aprocedure performed on a test sample, for subsequent application inassessing the characteristic of the etch process performed on othersamples.
 81. The method according to claim 80, wherein calibrating thethreshold level comprises making measurements of the etch indicatorsignal generated by the test sample, and comparing the measurements toat least one of a cross-sectional profile of the contact openings in thetest sample and a conductivity of electrical contacts made through thecontact openings in the test sample.
 82. The method according to claim79, wherein the sample has a barrier layer formed between the first andsecond layers, and wherein finding the level of the etch indicatorsignal comprises finding a first level that corresponds to etching ofthe contact openings through the second layer to expose the barrierlayer, and finding a second level that corresponds to etching of thecontact openings through the barrier layer to expose the first layerwithin the openings.
 83. The method according to claim 82, whereincomparing the etch indicator signal comprises analyzing the etchindicator signal after etching the second layer in order to assess anintegrity of the barrier layer, and then analyzing the etch indicatorsignal after etching the barrier layer in order to verify that at leastsome of the contact openings have been etched through the barrier layerto the first layer.
 84. The method according to claim 77, and comprisingevaluating a variation of the etch indicator signal across the sample soas to assess a uniformity of the etch process.
 85. The method accordingto claim 84, wherein evaluating the variation comprises signaling that aprocess fault has occurred if the variation of the etch indicator signalacross the sample is greater than a predetermined maximum.
 86. Themethod according to claim 77, wherein the sample comprises asemiconductor wafer, and wherein the contact openings comprise at leastone of contact holes, trenches and vias.
 87. The method according toclaim 77, wherein the sample has a photoresist layer overlying thesecond layer, the photoresist layer having been used in etching thecontact openings, and wherein measuring the etch indicator signalcomprises measuring the etch indicator signal prior to removing thephotoresist layer.
 88. The method according to claim 87, and comprising,if the etch indicator signal indicates that a depth of one or more ofthe contact openings is less than a predetermined limit, further etchingthe second layer using the photoresist layer so as to increase thedepth.
 89. The method according to claim 77, wherein determining theetch indicator signal comprises measuring a beam current of the beam ofcharged particles, and analyzing a ratio of at least one of the specimencurrent and the total yield of the electrons to the beam current. 90.The method according to claim 77, wherein directing the beam comprisesdirecting a pulsed beam of the charged particles to irradiate thesample, and wherein determining the etch indicator signal comprisesmeasuring a time variation of the specimen current by capacitivecoupling to the sample.
 91. A method for process monitoring of a samplehaving a first layer that is at least partially conductive and a secondlayer formed over the first layer, wherein contact openings are formedin the second layer by an etch process, the method comprising: directinga beam of charged particles to irradiate each of a plurality of theopenings that share a given set of characteristics and are disposed atdifferent, respective positions across the sample; measuring at leastone of a specimen current flowing through the first layer and a totalyield of electrons emitted from a surface of the sample in response toirradiation of the contact openings by the beam of charged particles,thus producing an etch indicator signal as a function of the respectivepositions of the plurality of the openings; and evaluating a variationof the etch indicator signal across the sample so as to assess auniformity of the etch process.
 92. The method according to claim 91,wherein evaluating the variation comprises determining that a processfault has occurred if the variation of the etch indicator signal acrossthe sample is greater than a predetermined maximum.
 93. The methodaccording to claim 91, wherein the sample comprises a semiconductorwafer, and wherein the contact openings comprise at least one of contactholes, trenches and vias.
 94. The method according to claim 91, whereinevaluating the variation of the etch indicator signal comprisesmeasuring a beam current of the beam of charged particles, and analyzinga ratio of the etch indicator signal to the beam current.
 95. The methodaccording to claim 91, wherein directing the beam comprises directing apulsed beam of the charged particles to irradiate the sample, andwherein measuring the specimen current comprises measuring a timevariation of the specimen current by capacitive coupling to the sample.96. Apparatus for etching a sample having a first layer that is at leastpartially conductive and a second layer formed over the first layer,contact openings having been created in the second layer by an etchprocess, the contact openings including a plurality of test openingshaving different, respective transverse dimensions, the apparatuscomprising: a test station, which comprises: a particle beam source,which is adapted to direct a beam of charged particles to irradiate thetest openings; and a current measuring device, which is coupled tomeasure, in response to the beam, at least one of a specimen currentflowing through the first layer and a total yield of electrons emittedfrom a surface of the sample, thus producing an etch indicator signal;and a controller, which is adapted to analyze the etch indicator signalas a function of the transverse dimensions of the test openings so as toassess a characteristic of the etch process.
 97. The apparatus accordingto claim 96, wherein the controller is adapted to assess a residualthickness of the dielectric layer at a bottom of the test openings as afunction of the transverse dimensions.
 98. The apparatus according toclaim 97, wherein the test openings comprise a first opening having afirst transverse dimension, and at least a second opening having asecond transverse dimension that is less than the first transversedimension, and wherein the controller is adapted to control the etchprocess, in response to the etch indicator signal, so that the firstopening is sufficiently deep to reach the first layer, while at leastthe second opening is not sufficiently deep to reach the first layer.99. The apparatus according to claim 98, wherein the test openingsfurther comprise a third opening, having a third transverse dimensionintermediate the first and second transverse dimensions, and wherein thecontroller is adapted to detect a potential process defect when the etchindicator signal indicates that the third opening is not sufficientlydeep to reach the first layer.
 100. The apparatus according to claim 97,wherein the sample has a barrier layer formed between the first andsecond layers, and wherein the controller is adapted to analyze the etchindicator signal after etching of the second layer in order to assess anintegrity of the barrier layer, and to analyze the etch indicator signalafter etching of the barrier layer.
 101. The apparatus according toclaim 96, wherein the controller is adapted to assess a criticaldimension of a bottom of the test openings as a function of thetransverse dimensions.
 102. The apparatus according to claim 96, whereinthe current measuring device is further adapted to measure a beamcurrent of the beam of charged particles, and wherein the controller isadapted to analyze a ratio of the etch indicator signal to the beamcurrent.
 103. The apparatus according to claim 96, wherein the currentmeasuring device comprises a secondary electron detector, for detectingthe total yield of the electrons emitted from the surface of the sample,and a primary electron detector, for detecting a primary current of thebeam, and is wherein the controller is adapted to take a differencebetween the primary current and the total yield in order to determinethe etch indicator signal.
 104. The apparatus according to claim 96,wherein the plurality of test openings comprises multiple groups of thetest openings in respective test areas, which are distributed indifferent locations across the sample, and wherein the test stationcomprises a positioning device, which is adapted to position at leastone of the beam and the sample so as to irradiate each of at least twoof the test areas in turn.
 105. The apparatus according to claim 104,wherein the controller is adapted to evaluate a variation of the etchindicator signal across the sample so as to assess a uniformity of theetch process.
 106. The apparatus according to claim 96, wherein theplurality of test openings comprise at least first and second arrays oftest openings, characterized by different, respective first and secondspacings between the test openings in the arrays.
 107. The apparatusaccording to claim 96, wherein the beam source is adapted to irradiatethe test openings along a beam axis that deviates substantially in anglefrom a normal to a surface of the sample.
 108. The apparatus accordingto claim 107, wherein the test openings have side walls and a bottom,and wherein the beam axis is angled so that more of the chargedparticles strike the side walls than strike the bottom.
 109. Theapparatus according to claim 96, wherein the beam source is adapted toprecharge a surface of the sample in proximity to the test openings, soas to facilitate measurement of the specimen current by the currentmeasuring device.
 110. The apparatus according to claim 109, wherein thebeam causes electrons to be emitted from the surface, and wherein theapparatus comprises a bias electrode, which is positioned and coupled tocreate an electric field in a vicinity of the surface so as to cause atleast a portion of the emitted electrons to return to the surface,thereby generating a negative precharge at the surface.
 111. Theapparatus according to claim 96, wherein the sample comprises asemiconductor wafer, and the contact openings comprise at least one ofcontact holes, trenches and vias.
 112. The apparatus according to claim96, wherein the test station is adapted to receive the sample with aphotoresist layer overlying the second layer, the photoresist layerhaving been used in etching the contact openings, so as to measure atleast one of the specimen current and the total yield of the electronswhile irradiating the test area with the particle beam, prior toremoving the photoresist layer.
 113. The apparatus according to claim112, and comprising an etch station, which is adapted to form thecontact openings in the second layer by the etch process, wherein thecontroller is adapted to control the etch process, in response to theetch indicator signal, so as to cause the etch station to further etchthe second layer using the photoresist layer so as to increase a depthof the contact openings if the etch indicator signal indicates that aresidual thickness of the second layer at a bottom of one or more of thetest openings is greater than a predetermined limit.
 114. The apparatusaccording to claim 96, wherein the sample comprises a semiconductorwafer, and wherein at least some of the contact openings not comprisedin the plurality of test openings belong to multiple microelectroniccircuits on the wafer, wherein the circuits are separated by scribelines, and the test openings are located on one of the scribe lines.115. The apparatus according to claim 114, wherein the controller isadapted to analyze the etch indicator signal so as to detect a residuewithin the contact openings, and to control the particle beam source soas to irradiate the sample with the beam of charged particles in orderto remove the residue.
 116. The apparatus according to claim 96, whereinthe particle beam source is adapted to pulse the beam of the chargedparticles that irradiates the test openings, and wherein the currentmeasuring device is capacitively coupled to the sample so as to measurea time variation of the specimen current.
 117. Apparatus for etching asample having a first layer that is at least partially conductive and asecond layer formed over the first layer, contact openings having beencreated in the second layer by an etch process, the contact openingsincluding at least first and second arrays of test openings,characterized by different, respective first and second spacings betweenthe test openings in the first and second arrays, the apparatuscomprising: a test station, which comprises: a particle beam source,which is adapted to direct a beam of charged particles to irradiate thetest openings; and a current measuring device, which is coupled tomeasure, in response to the beam, at least one of a specimen currentflowing through the first layer and a total yield of electrons emittedfrom a surface of the sample, thus producing an etch indicator signal;and a controller, which is adapted to analyze the etch indicator signalas a function of the spacings of the arrays of the test openings so asto assess a characteristic of the etch process.
 118. The apparatusaccording to claim 117, wherein the controller is adapted to assess aresidual thickness of the dielectric layer at a bottom of the testopenings as a function of the spacings.
 119. The apparatus according toclaim 118, wherein the first spacing is substantially greater than thesecond spacing, and wherein the controller is adapted to control theetch process, in response to the etch indicator signal, so that the testopenings in the first array are sufficiently deep to reach the firstlayer, while the test openings in the second array are not sufficientlydeep to reach the first layer.
 120. The apparatus according to claim118, wherein the sample has a barrier layer formed between the first andsecond layers, and wherein the controller is adapted to analyze the etchindicator signal after etching of the second layer in order to assess anintegrity of the barrier layer, and to analyze the etch indicator signalafter etching of the barrier layer.
 121. The apparatus according toclaim 117, wherein the controller is adapted to assess a criticaldimension of a bottom of the test openings as a function of thetransverse dimensions.
 122. The apparatus according to claim 117,wherein the current measuring device is further adapted to measure abeam current of the beam of charged particles, and wherein thecontroller is adapted to analyze a ratio of the etch indicator signal tothe beam current.
 123. The apparatus according to claim 117, wherein thecurrent measuring device comprises a secondary electron detector, fordetecting the total yield of the electrons emitted from the surface ofthe sample, and a primary electron detector, for detecting a primarycurrent of the beam, and is adapted to take a difference between theprimary current and the total yield in order to determine the etchindicator signal.
 124. The apparatus according to claim 117, wherein thetest openings comprises multiple arrays of the test openings havingdifferent, respective spacings in a plurality of test areas, which aredistributed in different locations across the sample, and wherein thetest station comprises a positioning device, which is adapted toposition at least one of the beam and the sample so as to irradiate eachof at least two of the test areas in turn.
 125. The apparatus accordingto claim 24, wherein the controller is adapted to evaluate a variationof the etch indicator signal across the sample so as to assess auniformity of the etch process.
 126. The apparatus according to claim117, wherein the beam source is adapted to irradiate the test openingsalong a beam axis that deviates substantially in angle from a normal toa surface of the sample.
 127. The apparatus according to claim 126,wherein the test openings have side walls and a bottom, and wherein thebeam axis is angled so that more of the charged particles strike theside walls than strike the bottom.
 128. The apparatus according to claim117, wherein the beam source is adapted to precharge a surface of thesample in proximity to the test openings, so as to facilitatemeasurement of the specimen current by the current measuring device.129. The apparatus according to claim 128, wherein the beam causeselectrons to be emitted from the surface, and wherein the apparatuscomprises a bias electrode, which is positioned and coupled to create anelectric field in a vicinity of the surface so as to cause at least aportion of the emitted electrons to return to the surface, therebygenerating a negative precharge at the surface.
 130. The apparatusaccording to claim 117, wherein the sample comprises a semiconductorwafer, and the contact openings comprise at least one of contact holes,trenches and vias.
 131. The apparatus according to claim 117, whereinthe test station is adapted to receive the sample with a photoresistlayer overlying the second layer, the photoresist layer having been usedin etching the contact openings, so as to measure at least one of thespecimen current and the total yield of the electrons while irradiatingthe test area with the particle beam, prior to removing the photoresistlayer.
 132. The apparatus according to claim 131, and comprising an etchstation, which is adapted to form the contact openings in the secondlayer by the etch process, wherein the controller is adapted to controlthe etch process, in response to the etch indicator signal, so as tocause the etch station to further etch the second layer using thephotoresist layer so as to increase a depth of the contact openings ifthe etch indicator signal indicates that a residual thickness of thesecond layer at a bottom of one or more of the test openings is greaterthan a predetermined limit.
 133. The apparatus according to claim 117,wherein the sample comprises a semiconductor wafer, and wherein at leastsome of the contact openings not comprised in the plurality of testopenings belong to multiple microelectronic circuits on the wafer,wherein the circuits are separated by scribe lines, and the testopenings are located on one of the scribe lines.
 134. The apparatusaccording to claim 133, wherein the controller is adapted to analyze theetch indicator signal so as to detect a residue within the contactopenings, and to control the particle beam source so as to irradiate thesample with the beam of charged particles in order to remove theresidue.
 135. The apparatus according to claim 117, wherein the particlebeam source is adapted to pulse the beam of the charged particles thatirradiates the test openings, and wherein the current measuring deviceis capacitively coupled to the sample so as to measure a time variationof the specimen current.
 136. Apparatus for monitoring a process carriedout on a sample, the apparatus comprising: a particle beam source, whichis adapted to direct a beam of charged particles to irradiate the samplealong a beam axis that deviates substantially in angle from a normal toa surface of the sample; a current measuring device, which is coupled tomeasure, in response to the beam, a specimen current flowing through thesample; and a controller, which is adapted to analyze the specimencurrent so as to assess a characteristic of the etch process.
 137. Theapparatus according to claim 136, wherein the sample has a first layerthat is at least partially conductive and a second layer formed over thefirst layer, and wherein the process comprises an etch process, which isapplied to the sample so as to produce contact openings in the secondlayer, and wherein directing the beam comprises irradiating the contactopenings, and wherein the controller is adapted to assess the etchprocess by analyzing the specimen current.
 138. The apparatus accordingto claim 137, wherein the sample has a barrier layer formed between thefirst and second layers, and wherein the controller is adapted toanalyze the specimen current after etching of the second layer in orderto assess an integrity of the barrier layer, and to analyze the specimencurrent after etching of the barrier layer.
 139. The apparatus accordingto claim 136, wherein some of the contact holes are characterized by atilt relative to the normal to the surface, and wherein the beam isangled so as to compensate for the tilt.
 140. The apparatus according toclaim 136, wherein the contact openings have side walls and a bottom,and wherein the beam is angled so that more of the charged particlesstrike the side walls than strike the bottom.
 141. The apparatusaccording to claim 136, wherein the contact openings are characterizedby an aspect ratio, and wherein the beam axis is aligned at an anglethat deviates from the normal to the surface by at least an arctangentof an inverse of the aspect ratio.
 142. The apparatus according to claim136, wherein the controller is adapted to assess, based on the specimencurrent, whether a contaminant residue is present within the contactopenings.
 143. The apparatus according to claim 142, wherein theparticle beam source is further adapted to irradiate the sample with thebeam of charged particles along the normal to the surface so as toremove the residue.
 144. The apparatus according to claim 136, whereinthe particle beam is adapted to negatively precharge the surface of thesample in proximity the contact openings, so as to facilitatemeasurement of the specimen current by the current measuring device.145. The apparatus according to claim 144, wherein the beam causeselectrons to be emitted from the surface, and wherein the apparatuscomprises a bias electrode, which is positioned and coupled to create anelectric field in a vicinity of the surface, so as to cause at least aportion of the emitted electrons to return to the surface, therebygenerating a negative precharge at the surface.
 146. The apparatusaccording to claim 136, wherein the sample comprises a semiconductorwafer, and wherein the contact openings comprise at least one of contactholes, trenches and vias.
 147. The apparatus according to claim 136,wherein the current measuring device is further adapted to measure abeam current of the beam of charged particles, and wherein thecontroller is adapted to analyze a ratio of the specimen current to thebeam current.
 148. The apparatus according to claim 136, wherein theparticle beam source is adapted to pulse the beam of the chargedparticles that irradiates the test openings, and wherein the currentmeasuring device is capacitively coupled to the sample so as to measurea time variation of the specimen current.
 149. Apparatus for processmonitoring, comprising: a particle beam source, which is adapted todirect a beam of charged particles to irradiate a surface of a sample,whereby electrons are emitted from the surface; a bias electrode, whichis adapted to apply an electric field in a vicinity of the surface, soas to cause at least a portion of the electrons emitted during theprecharging interval to return to the surface, thereby generating anegative precharge at the surface; and a current measuring device, whichis coupled to receive a signal produced by the sample in response to thebeam and the negative precharge.
 150. The apparatus according to claim149, wherein the sample has a first layer that is at least partiallyconductive and a second layer formed over the first layer, and whereinthe negative precharge is formed on the surface of the second layer.151. The apparatus according to claim 150, wherein the second layercomprises a dielectric material.
 152. The apparatus according to claim149, wherein the current measuring device is adapted to measure at leastone of a specimen current flowing through the sample and a total yieldof electrons emitted from the surface of the sample.
 153. The apparatusaccording to claim 152, wherein the sample has a first layer that is atleast partially conductive and a second layer formed over the firstlayer, with contact openings formed in the second layer by an etchprocess, and wherein the signal is indicative of a characteristic of theetch process.
 154. The apparatus according to claim 153, wherein thesample comprises a semiconductor wafer, and wherein the contact openingscomprise at least one of contact holes, trenches and vias.
 155. Theapparatus according to claim 153, wherein the sample has a barrier layerformed between the first and second layers, and wherein the signalreceived by the current measuring device after etching of the secondlayer is indicative of an integrity of the barrier layer, and the signalreceived by the current measuring device after etching of the barrierlayer is indicative of an extent to which the contact openings have beenetched through the barrier layer to the first layer.
 156. The apparatusaccording to claim 149, wherein the beam source is adjustable to producethe beam with first beam characteristics during a precharging intervalso as to generate the negative precharge at the surface, and then toproduce the beam after the precharging interval with second beamcharacteristics, so as to generate the signal.
 157. The apparatusaccording to claim 156, wherein the beam source is adapted to irradiatethe surface with the particles during the precharging interval with anenergy in a positive charging domain of the surface of the sample, 158.Apparatus for testing a semiconductor device, comprising: a radiationsource, which is adapted to irradiate a junction in the semiconductordevice with a first beam comprising electromagnetic radiation; aparticle beam source, which is adapted to irradiate the device with asecond beam comprising charged particles, so that at least some of thecharged particles are incident on the junction substantiallysimultaneously with the electromagnetic radiation; and a measuringelement, which is adapted to measure, in response to incidence of thefirst and second beams on the junction, a property of the device. 159.The apparatus according to claim 158, wherein the measuring element isadapted to form an electronic image of the device in response toincidence of the first and second beams on the junction.
 160. Theapparatus according to claim 158, wherein the junction comprises asemiconductor material, and wherein the radiation source is adapted toirradiate the junction with photons having an energy greater than orequal to a bandgap of the semiconductor material.
 161. The apparatusaccording to claim 158, wherein the junction comprises a P-N junction.162. The apparatus according to claim 158, wherein the measuring elementcomprises a current measuring element, which is adapted to measure acurrent flowing through the device.
 163. The apparatus according toclaim 162, wherein a dielectric layer is formed over the junction, and acontact hole is formed through the dielectric layer in order to contactthe junction, and wherein the radiation source and particle beam sourceare respectively adapted to direct the first and second beams into thecontact hole, so that the measured current is indicative of acharacteristic of the contact hole.
 164. The apparatus according toclaim 163, wherein the measured current is indicative of whether thecontact hole is suitable to make a conductive electrical contact withthe junction.
 165. Apparatus for monitoring an etch process applied to asample having a first layer that is at least partially conductive and asecond layer formed over the first layer, following production ofcontact openings in the second layer by the etch process, the apparatuscomprising: a particle beam source, which is adapted to direct a beam ofcharged particles to irradiate one or more of the contact openings; abeam current detector, for detecting a primary current of the beam; asecondary electron detector, for detecting a total yield of electronsemitted from a surface of the sample in response incidence of the beamon the contact openings; and a controller, which is adapted a relationbetween the primary current and the total yield of the electrons so asto assess a characteristic of the etch process.
 166. The apparatusaccording to claim 165, wherein the relation comprises a differencebetween the primary current and the total yield.
 167. The apparatusaccording to claim 165, wherein the relation comprises a ratio betweenthe primary current and the total yield.
 168. The apparatus according toclaim 165, wherein the particle beam source is adapted to irradiatemultiple contact openings, which are distributed in different locationsacross the sample, and wherein the controller is adapted to evaluate avariation of the relation across the sample so as to assess a uniformityof the etch process.
 169. The apparatus according to claim 165, whereinthe particle beam source is adapted to precharge a surface of the samplein proximity the one or more of the contact openings, so that thesecondary electron detector measures the total yield of the electronsemitted from the precharged surface.
 170. The apparatus according toclaim 165, wherein the sample comprises a semiconductor wafer, andwherein the contact openings comprise at least one of contact holes,trenches and vias.
 171. The apparatus according to claim 165, whereinthe sample has a barrier layer formed between the first and secondlayers, and wherein the controller is adapted to analyze the relationafter etching of the second layer in order to assess an integrity of thebarrier layer, and to analyze the relation after etching of the barrierlayer.
 172. Apparatus for monitoring a process applied to a samplehaving a first layer that is at least partially conductive and a secondlayer formed over the first layer, contact openings having been createdin the second layer by an etch process, the apparatus comprising: a teststation, comprising: a particle beam source, which is adapted to directa beam of charged particles to irradiate each of a plurality of thecontact openings that are disposed at different, respective positionsover a surface of the sample; and a current measuring device, which isadapted to produce an etch indicator signal by measuring, in response toirradiation of each of the plurality of the contact openings by the beamof charged particles, at least one of a specimen current flowing throughthe first layer and a total yield of electrons emitted from a surface ofthe sample; and a controller, which is adapted to store a calibratedthreshold level of the etch indicator signal for a given set ofproperties of the etch process, and to compare the respective etchindicator signal produced with respect to each of the plurality of thecontact openings to the threshold level so as to assess a characteristicof the etch process.
 173. The apparatus according to claim 172, whereinthe controller is adapted to determine, if an absolute magnitude of thespecimen current falls below the threshold level by more than apredetermined margin, that at least some of the contact openings areunderetched.
 174. The apparatus according to claim 173, wherein thethreshold level is calibrated by finding the level of the etch indicatorsignal that corresponds to etching of the contact openings through thesecond layer to expose the first layer within the opening.
 175. Theapparatus according to claim 174, wherein the threshold level iscalibrated in a procedure performed on a test sample, for subsequentapplication in assessing the characteristic of the etch processperformed on other samples.
 176. The apparatus according to claim 175,wherein the threshold level is calibrated making measurements of theetch indicator signal generated by the test sample, and comparing themeasurements to at least one of a cross-sectional profile of the contactopenings in the test sample and a conductivity of electrical contactsmade through the contact openings in the test sample.
 177. The apparatusaccording to claim 172, wherein the controller is further adapted toanalyze a variation of the etch indicator signal across the sample so asto assess a uniformity of the etch process.
 178. The apparatus accordingto claim 177, wherein the controller is adapted to signal that a processfault has occurred if the variation of the etch indicator signal acrossthe sample is greater than a predetermined maximum.
 179. The apparatusaccording to claim 172, wherein the sample comprises a semiconductorwafer, and wherein the contact openings comprise at least one of contactholes, trenches and vias.
 180. The apparatus according to claim 172,wherein the sample has a photoresist layer overlying the second layer,which is used in etching the contact openings, and wherein the teststation is adapted to measure the etch indicator signal prior toremoving the photoresist layer.
 181. The apparatus according to claim180, and comprising an etch station, which is adapted to form thecontact openings in the second layer by the etch process, wherein thecontroller is adapted to cause the etch station to further etch thesecond layer using the photoresist layer so as to increase a depth ofone or more of the contact openings, if the etch indicator signalindicates that the depth is less than a predetermined limit.
 182. Theapparatus according to claim 172, wherein the current measuring deviceis further adapted to measure a beam current of the beam of chargedparticles, and wherein the control is adapted to analyze a ratio of atleast one of the specimen current and the total yield of the electronsto the beam current.
 183. The apparatus according to claim 172, whereinthe particle beam source is adapted to pulse the beam of the chargedparticles that irradiates the sample, and wherein the current measuringdevice is capacitively coupled to the sample so as to measure a timevariation of the specimen current.
 184. The apparatus according to claim172, wherein the sample has a barrier layer formed between the first andsecond layers, and wherein the apparatus comprises an etch station,which is adapted to form the contact openings in the second layer by theetch process and to etch the contact openings through the barrier layer,and wherein the controller is adapted to store first and secondcalibrated threshold levels of the etch indicator signal and to comparethe respective etch indicator signal to the first calibrated thresholdlevel after the etch station has etched the contact openings through thesecond layer, and to the second calibrated threshold level after theetch station has etched the contact openings through the barrier layer.185. The apparatus according to claim 184, wherein the first calibratedthreshold level corresponds to etching of the contact openings throughthe second layer to expose the barrier layer, and the second calibratedthreshold level corresponds to etching of the contact openings throughthe barrier layer to expose the first layer within the openings, andwherein the controller is adapted to compare the etch indicator signalto the first calibrated threshold level after etching of the secondlayer in order to assess an integrity of the barrier layer, and tocompare the etch indicator signal to the second calibrated thresholdlevel after etching of the barrier layer in order to verify that atleast some of the contact openings have been etched through the barrierlayer to the first layer.
 186. Apparatus for monitoring a processapplied to a sample having a first layer that is at least partiallyconductive and a second layer formed over the first layer, contactopenings having been formed in the second layer by an etch process, theapparatus comprising: a test station, which comprises: a particle beamsource, which is adapted to direct a beam of charged particles toirradiate each of a plurality of the openings that are disposed atdifferent, respective positions across the sample; and a currentmeasuring device, which is adapted to measure at least one of a specimencurrent flowing through the first layer and a total yield of electronsemitted from a surface of the sample in response to irradiation of thecontact openings by the beam of charged particles, thus producing anetch indicator signal as a function of the respective positions of theplurality of the openings; and a controller, which is adapted toevaluate a variation of the etch indicator signal across the sample soas to assess a uniformity of the etch process.
 187. The apparatusaccording to claim 186, wherein the controller is adapted to determinethat a process fault has occurred if the variation of the etch indicatorsignal across the sample is greater than a predetermined maximum. 188.The apparatus according to claim 186, wherein the sample comprises asemiconductor wafer, and wherein the contact openings comprise at leastone of contact holes, trenches and vias.
 189. The apparatus according toclaim 186, wherein the current measuring device is further adapted tomeasure a beam current of the beam of charged particles, and wherein thecontroller is adapted to analyze a ratio of the etch indicator signal tothe beam current.
 190. The apparatus according to claim 186, wherein theparticle beam source is adapted to pulse the beam of the chargedparticles that irradiates the sample, and wherein the current measuringdevice is capacitively coupled to the sample so as to measure a timevariation of the specimen current.
 191. A method for process monitoringof a sample having a first layer that is at least partially conductive,a second, barrier layer formed over the first layer, and a third,dielectric layer formed over the second layer, the method comprising:etching contact openings in the third layer in a first etch process;directing a beam of charged particles to irradiate the contact openingsin a first monitoring step following the first etch process; measuringat least one of a specimen current flowing through the first layer and atotal yield of electrons emitted from a surface of the sample inresponse to irradiation of the contact openings by the beam of chargedparticles in the first monitoring step, thus producing a second etchindicator signal; evaluating the first etch indicator signal to assess afirst characteristic of the first etch process; further etching thecontact openings from the third layer into the second layer in a secondetch process; directing the beam of charged particles to irradiate thecontact openings in a second monitoring step following the second etchprocess; measuring the at least one of the specimen current flowingthrough the first layer and the total yield of the electrons emittedfrom the surface of the sample in response to irradiation of the contactopenings by the beam of charged particles in the second monitoring step,thus producing a second etch indicator signal; and evaluating the secondetch indicator signal to assess a second characteristic of the secondetch process.
 192. The method according to claim 191, wherein evaluatingthe first etch indicator signal comprises assessing an integrity of thesecond layer.
 193. The method according to claim 191, wherein evaluatingthe second etch indicator signal comprises verifying that at least someof the contact openings have been etched through the second layer to thefirst layer.
 194. Apparatus for process monitoring of a sample having afirst layer that is at least partially conductive, a second, barrierlayer formed over the first layer, and a third, dielectric layer formedover the second layer, the apparatus comprising: an etch station, whichis adapted to form contact openings in the third layer in a first etchprocess, and subsequently to further etch the contact openings from thethird layer into the second layer in a second etch process; a teststation, which comprises: a particle beam source, which is adapted todirect a beam of charged particles to irradiate the contact openings;and a current measuring device, which is adapted to measure at least oneof a specimen current flowing through the first layer and a total yieldof electrons emitted from a surface of the sample in response toirradiation of the contact openings by the beam of charged particles,thus producing a first etch indicator signal following the first etchprocess and a second etch indicator signal following the second etchprocess; and a controller, which is adapted to evaluate the first etchindicator signal in order to assess a first characteristic of the firstetch process and to evaluate the second etch indicator signal in orderto assess a second characteristic of the second etch process.
 195. Theapparatus according to claim 194, wherein the controller is adapted toassess, in response to the first etch indicator signal, an integrity ofthe second layer.
 196. The apparatus according to claim 194, wherein thecontroller is adapted to verify, in response to the second etchindicator signal, that at least some of the contact openings have beenetched through the second layer to the first layer.